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Automatic devices' resolution in nuclear emulsion observation
656
Nuclear Instruments and Methods in Physics Research 225 I i9~4) 656 66(I North-Holland. Amstcrdam
A U T O M A T I C DEVICES' R E S O L U T I O N IN NUCLEAR E M U L S I O N O B S E R V A T I O N G. B A R O N I , A.M. C E C C H E T T I , S. D E L L ' U O M O , S. D I L I B E R T O , P. G I N O B B I , E. L A M A N N A , A. M A N F R E D I N I , M.A. M A Z Z O N I , F. M E D D I , S. P E T R E R A , G. R O M A N O , G. R O S A a n d C. S G A R B I Dipartimento di Fisica, Universit& "La Sapienza'" Roma, Italy and Sezione INFN, Roma, Italy
Some results on the precision obtainable using automatic devices in nuclear emulsion measurements and simplified methods for their application in standard work are presented.
1. Introduction The realization of automatic devices, especially dedicated to nuclear emulsion work [1], speeds up considerably the scanning and the measurements of selected events. In this report we summarize our scanning method already published [1] and we study: 1) the accuracy obtainable with these devices, in comparison with direct measurements in order to see if, at least in a considerable number of cases, these devices are adequate for reliable events measurements; 2) the efficiency in track deflection detection as a function of scattering angle with no time consuming methods; 3) the efficiency, in standard routine work, in very short decay path detection, i.e. paths shorter than the dimensions of the confusion volume. With this study, we check if the devices and the elaboration methods that we have prepared in Rome are adequate for beauty detection in view of their application in the two CERN experiments: WA71 (collaboration: CERN, Genoa, Milan, Moscow, Paris, Rome, Santander and Valencia) and WA75 (collaboration: Bari, Bruxelles, CERN, Dublin, Karya, Kobe, London, Nagoya, Rome, Turin, Utsunomiya and Yokohama).
the original g a i n s dimensions that dominate the dispersion values of grains around the track. This dimension, as the original grains spacing, is of the order of 0.1 #m. The developed grains have diameters of 0.4-0.6/~m and spacings of 2-8/~m for M.I.P. tracks. Table 1 shows, at different magnifications, the instrumental sensitivity of this apparatus when used for data acquisition in the field of view and the precision obtained in single grain repeated measurements. Table 2 shows the results of the fit of beam tracks as straight lines in different exposure conditions. Vertical primaries are self-corrected for distortion [2].
KORISTKA R4 MICROSCOPE
2. Automatic devices
The devices we use for this study are a TV digitized system (fig. 1) and a Leitz microscope equipped with a Z encoder and with a camera lucida for the superimposition to the microscope field of view of a digitized table with an acquisition coordinate cursor (fig. 2). The systems are interfaced to a PDP11/34 via a microprocessor based interface. Building this apparatus, we always bear in mind that the precisions to be reached have to be comparable with 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Fig. 1. Block diagram of the automatic TV system (microprocessor based) interactively connected to PDPt1/34. Man.agement via tracking ball or via software: (a) video analysis (digitized internal field of view, frame memory for external data); (b) coordinate acquisition (external xyz, internal xy).
657
G. Baroni et aL / A u t o m a t i c devices' resolution in nuclear emulsion
L[ITZ MICliOSCOP£ Z DISPLAY
CAMERALUCIDA
[
I
DIOITIZ[D TABLE X,Y)
X YZ INTERFACE
I
~ I'1
tHoootZN
PRINTER
T
Table 2 Fit of tracks as straight lines. (Ob. 40 × )
Y = A + BX
HOST-COMPUTER PDP 11/14
AA AB
Horizontal exp.
Vertical exp.
0.1 #m 3.3 mrad
1.2 /~m 3.7 mrad
VIDEO TERMINAL
Fig. 2. Block diagram of HP system (microprocessor based" connected to PDPll/34.
Fig. 3 shows the grains dispersion distribution with respect to the fitted track, with 40 × magnification, for horizontal and vertical primaries. The accuracy, even at a magnification lower than the highest possible, is really adequate not only for scanning and detection work but for events measurements too. In fact angles measured with errors of 3 - 4 mrad and positions with errors of 1 # m seem to us to be adequate in standard work. The error of 0.1-0.2 # m obtainable at the higher magnification appears to be completely satisfactory, even when compared with the values of 0.1 mrad and of 0.03 # m which we can reach in direct and very accurate microscope observation (but with a considerable effort and spending a lot of time). It should be mentioned: the measurements acquisition with an automatic device is so quick that the correction for thermal drift (for a good microscope: 0.5 # m / h ) is not relevant and in any case can be made on line by a program.
in order to employ emulsions in a useful way, they are coupled to some external detectors allowing event selection and providing their features and position within a fiducial volume. In these experiments, emulsions need a special preparation for exposure in stackholders and with external apparatus, so that the scanning can be limited, in present generation hybrid experiments, to 0.1-0.2 m m 2 / e v e n t (typically the ratio s c a n n e d / e x posed volume is 10-4). Our devices are equipped with a scanning program that performs in sequence the following steps: it searches the starting point in the field of view, moves the microscope stage following a snail path, explores the whole (or partial) plate thickness and so enlarges the scanned area to 1, 2 and 3 standard deviations, allows the vertex coordinates acquisition when a star is found and a comparison with the external detector's data. All these
vertical axposurl
h0rixontal Ixpolure
10 OC
3. Application of automatic devices in detection of short living particles decay The emulsion's high spatial resolution makes these detectors exceptionally useful in detection of short decay path. In charm or beauty research, we are faced with very short decay paths and with small cross sections. Thus, Table 1 Magnification TVS: Sensitivity [#m/dig] Precision [/*m] HPS: Sensitivity [#m/dig] Precision [#m]
X Y Z X,Y Z X,Y Z X,Y Z
10 x
40 ×
100 ×
2.64 1.85 0.50 0.26 0.30 1.02 0.50 -
0.71 0.50 0.50 0.18 0.33 0.25 0.50 0.28 0.47
0.27 0.19 0.50 0.05 0.17 0.10 0.50 0.10 0.59
_ .. 2. ". . .1.a
-.e
. . . . . -4 0
q'-t_,_ . . . . 4
.s
12
.
. ~a
.
.
.
.
. -s
-4
, , , , 0 .4
#t m
Fig. 3. Grains distribution with respect to the fitted tracks for horizontal and vertical primaries (ob. 40 x ). VII. VISUAL DEVICES
658
G. Baroni et al. / Automatic devices' resolution in nuclear emulsion
operations require about 15 min if the primaries enter parallel to the plate surface and 10 min if the entrance direction is orthogonal to the plate surface. The combined effect of hybrid experiments and automatic devices produces a remarkable reduction of the scanning effort, so that now the analysis time is no longer negligible with respect to the scanning time, as in traditional emulsion work. We are even studying the possibihty of reducing, without losing accuracy, the time for analysis, consisting essentially in track following and in neutral search. The situations we face during the analysis are different, depending on whether we have to search charged (one prong, three or more prongs) or neutral decays whether these decays he inside or outside the star core, near the parent star or not, or whether the primaries enter parallel or normal to the emulsion surface. Of course there are no special problems in detecting multiprong charged d e c a y s far from the interaction vertex. Problems arise when one has to detect charged single prong decays that can escape in direct observation or when the decay arises inside the confusion volume downstream to the vertex. There are other not easy cases, as for example neutral decay detection, but
we have confined ourselves, for the moment, to studying only these two cases.
4. Shortest detectable distance
In direct microscope observation, the full detection efficiency is reached at a distance from the star centre of the order of 10-20 # m in horizontal exposure and of 30-80 # m in the vertical one. These figures depend strongly on the incident particles (neutrinos, photons, hadrons), on their energy and on the quality of the emulsions. It is possible, with very accurate measurements of the impact parameter of the tracks of the star, to detect shorter decay paths, but at the expense of great effort, so that these methods cannot be used in standard emulsion work [3]. We have analysed beauty and charm events generated by a Monte Carlo (20% diffractive and 80% central production) with phase space decay and the shortest expected beauty hfetime (10 -14 s) [4]. The results on expected impact parameters are shown in fig. 4; thus it seems that even with fighter but quicker emulsion work it is possible to mark the decays inside the analysed events.
% 100"
70 - 80 60 -- 70
/~ m
50 -- 60
40--
50
30-
40
20 -- 30
10-20
b
~m
Fig. 4. MONTECARLO results. Probability to have a beauty decay track with impact parameter higher than b for different values of the decay path from 10 to 80 pro.
659
G. Baroni et al. / Automatic devices' resolution in nuclea[ emulsion parameter
Impact
5. Detection of charged single prong decay
n.tr.
Vertical
exposure
The detection of single scatterings, when the scattering angle is smaller than 30-40 mrad, requires careful and continuous attention in direct observation: in order to improve their detection, without great effort, we can attempt to use the automatic devices measuring all the event tracks, picking up only few grains in different track segments at separations of a few hundred micrometres. A limitation of the method arises from the measurement errors. We have evaluated the order of magnitude of these errors, by measuring systematically the same track in two positions separated by 300 /tin (10 grains picked up at each position). Fig. 6 shows the distribution of the angular difference between two segments of the same track for horizontal and vertical plates respectively. For comparison, fig. 7 shows the same distribution as obtained from the Monte Carlo simulation for beauty decays. Thus single decays are in a condition to be detected in more than 95% of the cases in the horizontal exposure and in 85% of the cases in the vertical one.
200
100
l
1
o.'s
6. Conclusions With our automatic devices, TV and HP, we have already demonstrated that the scanning effort can be considerably reduced.
1~o
1.'5
vertical 2.0
~tm
Fig. 5. Experimental distribution of the impact parameter, with respect to the vertex, of tracks produced by high energy pion interactions in vertically exposed emulsions.
exposure
horizontal
exposure
70
60
50
In order to evaluate the possible background contribution in simplified analysis, we measured, with our automatic devices, the impact parameter of tracks produced in the interactions of pions of energies from 27 to 370 GeV, both in horizontally and vertically exposed plates. The results obtained picking up no more than 10 grains per track are shown in fig. 5. F r o m these distributions, we can deduce that, due to measurement errors, background tracks can simulate impact parameters up to 2.5 # m in the vertical exposure, where the confusion zone is more relevant. Thus the detection of impact parameters higher than this value could mark the presence of a charm or beauty decay. By a comparison of figs. 4 and 5, we can deduce that, with this analysis, beauties decaying at distances as low as 40 # m are in conditions to be detected, even in the vertical exposure, in more than 50% of the cases.
30
20
10
6
12
18
24
0
6
12
18
m
rad
Fig. 6. Experimental distributions of the angle differences between two segments of the same track. 10 grains picked up at each position 300 #m from one another. VII. VISUAL DEVICES
660
G, Baroni~et al. / Automatic devices' resolution in nuclear emulsion
900
n.tr.
\ 400
LL \
L
\ r...L__~
lO0
200
300
400
.'~, ( m r a d )
Fig. 7. MONTECARLO SIMULATION. Angular distribution of beauties decaying in a single charged prong.
I n this report, we have s h o w n that the accuracy o b t a i n a b l e with these devices is completely satisfactory even w h e n c o m p a r e d with very sophisticated direct m e a s u r e m e n t s a n d that it is possible to envisage simplified time saving analysis m e t h o d s with a d e q u a t e precision, allowing the i m p r o v e m e n t of the detection of single p r o n g decays or of very short p a t h decays. Of course, with these simplified methods, we do not use all the i n f o r m a t i o n stored in the emulsions which r e m a i n s available, for m e a s u r e m e n t s of special events. W e t h a n k Mrs C. Cattaneo, M r F. Cortellessa and M r s M. Federici for their careful work with these new apparatus.
References [1] G. Baroni et al., Nucl. Instr. and Meth. 214 (1983) 381; K. Niu: Proc 10th Int. Conf. on High Energy Phys., Madison, "Wisconsin (1980); W. Heinrich et al., Nucl. Tracks 5 (1981) 406; S. Tentindo and G. Vanderaeghe, CERN Report EPCEQ-IR1 (1981). [2] G. Baroni et al., Nota Interna n. 787, Istituto di Fisica, Universith di Roma (1981). [3] J.H. Shwe, F.H. Smith and W.H. Barkas, Phys. Rev. 125 (1962) 1024; G. Baroni, S. Di Liberto, P. Ginobbi, S. Petrera and G. Romano, Lett. Nuovo Cimento 242 (1979) 45; S. Petrera and G. Romano: Nucl. Instr. and Meth. 174 (1980) 53. [4] M.K. Gaillard and L. Maiani, Carges Inst. (1979) LAPP TH-09 C.
Nuclear Instruments and Methods in Physics Research 225 I i9~4) 656 66(I North-Holland. Amstcrdam
A U T O M A T I C DEVICES' R E S O L U T I O N IN NUCLEAR E M U L S I O N O B S E R V A T I O N G. B A R O N I , A.M. C E C C H E T T I , S. D E L L ' U O M O , S. D I L I B E R T O , P. G I N O B B I , E. L A M A N N A , A. M A N F R E D I N I , M.A. M A Z Z O N I , F. M E D D I , S. P E T R E R A , G. R O M A N O , G. R O S A a n d C. S G A R B I Dipartimento di Fisica, Universit& "La Sapienza'" Roma, Italy and Sezione INFN, Roma, Italy
Some results on the precision obtainable using automatic devices in nuclear emulsion measurements and simplified methods for their application in standard work are presented.
1. Introduction The realization of automatic devices, especially dedicated to nuclear emulsion work [1], speeds up considerably the scanning and the measurements of selected events. In this report we summarize our scanning method already published [1] and we study: 1) the accuracy obtainable with these devices, in comparison with direct measurements in order to see if, at least in a considerable number of cases, these devices are adequate for reliable events measurements; 2) the efficiency in track deflection detection as a function of scattering angle with no time consuming methods; 3) the efficiency, in standard routine work, in very short decay path detection, i.e. paths shorter than the dimensions of the confusion volume. With this study, we check if the devices and the elaboration methods that we have prepared in Rome are adequate for beauty detection in view of their application in the two CERN experiments: WA71 (collaboration: CERN, Genoa, Milan, Moscow, Paris, Rome, Santander and Valencia) and WA75 (collaboration: Bari, Bruxelles, CERN, Dublin, Karya, Kobe, London, Nagoya, Rome, Turin, Utsunomiya and Yokohama).
the original g a i n s dimensions that dominate the dispersion values of grains around the track. This dimension, as the original grains spacing, is of the order of 0.1 #m. The developed grains have diameters of 0.4-0.6/~m and spacings of 2-8/~m for M.I.P. tracks. Table 1 shows, at different magnifications, the instrumental sensitivity of this apparatus when used for data acquisition in the field of view and the precision obtained in single grain repeated measurements. Table 2 shows the results of the fit of beam tracks as straight lines in different exposure conditions. Vertical primaries are self-corrected for distortion [2].
KORISTKA R4 MICROSCOPE
2. Automatic devices
The devices we use for this study are a TV digitized system (fig. 1) and a Leitz microscope equipped with a Z encoder and with a camera lucida for the superimposition to the microscope field of view of a digitized table with an acquisition coordinate cursor (fig. 2). The systems are interfaced to a PDP11/34 via a microprocessor based interface. Building this apparatus, we always bear in mind that the precisions to be reached have to be comparable with 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Fig. 1. Block diagram of the automatic TV system (microprocessor based) interactively connected to PDPt1/34. Man.agement via tracking ball or via software: (a) video analysis (digitized internal field of view, frame memory for external data); (b) coordinate acquisition (external xyz, internal xy).
657
G. Baroni et aL / A u t o m a t i c devices' resolution in nuclear emulsion
L[ITZ MICliOSCOP£ Z DISPLAY
CAMERALUCIDA
[
I
DIOITIZ[D TABLE X,Y)
X YZ INTERFACE
I
~ I'1
tHoootZN
PRINTER
T
Table 2 Fit of tracks as straight lines. (Ob. 40 × )
Y = A + BX
HOST-COMPUTER PDP 11/14
AA AB
Horizontal exp.
Vertical exp.
0.1 #m 3.3 mrad
1.2 /~m 3.7 mrad
VIDEO TERMINAL
Fig. 2. Block diagram of HP system (microprocessor based" connected to PDPll/34.
Fig. 3 shows the grains dispersion distribution with respect to the fitted track, with 40 × magnification, for horizontal and vertical primaries. The accuracy, even at a magnification lower than the highest possible, is really adequate not only for scanning and detection work but for events measurements too. In fact angles measured with errors of 3 - 4 mrad and positions with errors of 1 # m seem to us to be adequate in standard work. The error of 0.1-0.2 # m obtainable at the higher magnification appears to be completely satisfactory, even when compared with the values of 0.1 mrad and of 0.03 # m which we can reach in direct and very accurate microscope observation (but with a considerable effort and spending a lot of time). It should be mentioned: the measurements acquisition with an automatic device is so quick that the correction for thermal drift (for a good microscope: 0.5 # m / h ) is not relevant and in any case can be made on line by a program.
in order to employ emulsions in a useful way, they are coupled to some external detectors allowing event selection and providing their features and position within a fiducial volume. In these experiments, emulsions need a special preparation for exposure in stackholders and with external apparatus, so that the scanning can be limited, in present generation hybrid experiments, to 0.1-0.2 m m 2 / e v e n t (typically the ratio s c a n n e d / e x posed volume is 10-4). Our devices are equipped with a scanning program that performs in sequence the following steps: it searches the starting point in the field of view, moves the microscope stage following a snail path, explores the whole (or partial) plate thickness and so enlarges the scanned area to 1, 2 and 3 standard deviations, allows the vertex coordinates acquisition when a star is found and a comparison with the external detector's data. All these
vertical axposurl
h0rixontal Ixpolure
10 OC
3. Application of automatic devices in detection of short living particles decay The emulsion's high spatial resolution makes these detectors exceptionally useful in detection of short decay path. In charm or beauty research, we are faced with very short decay paths and with small cross sections. Thus, Table 1 Magnification TVS: Sensitivity [#m/dig] Precision [/*m] HPS: Sensitivity [#m/dig] Precision [#m]
X Y Z X,Y Z X,Y Z X,Y Z
10 x
40 ×
100 ×
2.64 1.85 0.50 0.26 0.30 1.02 0.50 -
0.71 0.50 0.50 0.18 0.33 0.25 0.50 0.28 0.47
0.27 0.19 0.50 0.05 0.17 0.10 0.50 0.10 0.59
_ .. 2. ". . .1.a
-.e
. . . . . -4 0
q'-t_,_ . . . . 4
.s
12
.
. ~a
.
.
.
.
. -s
-4
, , , , 0 .4
#t m
Fig. 3. Grains distribution with respect to the fitted tracks for horizontal and vertical primaries (ob. 40 x ). VII. VISUAL DEVICES
658
G. Baroni et al. / Automatic devices' resolution in nuclear emulsion
operations require about 15 min if the primaries enter parallel to the plate surface and 10 min if the entrance direction is orthogonal to the plate surface. The combined effect of hybrid experiments and automatic devices produces a remarkable reduction of the scanning effort, so that now the analysis time is no longer negligible with respect to the scanning time, as in traditional emulsion work. We are even studying the possibihty of reducing, without losing accuracy, the time for analysis, consisting essentially in track following and in neutral search. The situations we face during the analysis are different, depending on whether we have to search charged (one prong, three or more prongs) or neutral decays whether these decays he inside or outside the star core, near the parent star or not, or whether the primaries enter parallel or normal to the emulsion surface. Of course there are no special problems in detecting multiprong charged d e c a y s far from the interaction vertex. Problems arise when one has to detect charged single prong decays that can escape in direct observation or when the decay arises inside the confusion volume downstream to the vertex. There are other not easy cases, as for example neutral decay detection, but
we have confined ourselves, for the moment, to studying only these two cases.
4. Shortest detectable distance
In direct microscope observation, the full detection efficiency is reached at a distance from the star centre of the order of 10-20 # m in horizontal exposure and of 30-80 # m in the vertical one. These figures depend strongly on the incident particles (neutrinos, photons, hadrons), on their energy and on the quality of the emulsions. It is possible, with very accurate measurements of the impact parameter of the tracks of the star, to detect shorter decay paths, but at the expense of great effort, so that these methods cannot be used in standard emulsion work [3]. We have analysed beauty and charm events generated by a Monte Carlo (20% diffractive and 80% central production) with phase space decay and the shortest expected beauty hfetime (10 -14 s) [4]. The results on expected impact parameters are shown in fig. 4; thus it seems that even with fighter but quicker emulsion work it is possible to mark the decays inside the analysed events.
% 100"
70 - 80 60 -- 70
/~ m
50 -- 60
40--
50
30-
40
20 -- 30
10-20
b
~m
Fig. 4. MONTECARLO results. Probability to have a beauty decay track with impact parameter higher than b for different values of the decay path from 10 to 80 pro.
659
G. Baroni et al. / Automatic devices' resolution in nuclea[ emulsion parameter
Impact
5. Detection of charged single prong decay
n.tr.
Vertical
exposure
The detection of single scatterings, when the scattering angle is smaller than 30-40 mrad, requires careful and continuous attention in direct observation: in order to improve their detection, without great effort, we can attempt to use the automatic devices measuring all the event tracks, picking up only few grains in different track segments at separations of a few hundred micrometres. A limitation of the method arises from the measurement errors. We have evaluated the order of magnitude of these errors, by measuring systematically the same track in two positions separated by 300 /tin (10 grains picked up at each position). Fig. 6 shows the distribution of the angular difference between two segments of the same track for horizontal and vertical plates respectively. For comparison, fig. 7 shows the same distribution as obtained from the Monte Carlo simulation for beauty decays. Thus single decays are in a condition to be detected in more than 95% of the cases in the horizontal exposure and in 85% of the cases in the vertical one.
200
100
l
1
o.'s
6. Conclusions With our automatic devices, TV and HP, we have already demonstrated that the scanning effort can be considerably reduced.
1~o
1.'5
vertical 2.0
~tm
Fig. 5. Experimental distribution of the impact parameter, with respect to the vertex, of tracks produced by high energy pion interactions in vertically exposed emulsions.
exposure
horizontal
exposure
70
60
50
In order to evaluate the possible background contribution in simplified analysis, we measured, with our automatic devices, the impact parameter of tracks produced in the interactions of pions of energies from 27 to 370 GeV, both in horizontally and vertically exposed plates. The results obtained picking up no more than 10 grains per track are shown in fig. 5. F r o m these distributions, we can deduce that, due to measurement errors, background tracks can simulate impact parameters up to 2.5 # m in the vertical exposure, where the confusion zone is more relevant. Thus the detection of impact parameters higher than this value could mark the presence of a charm or beauty decay. By a comparison of figs. 4 and 5, we can deduce that, with this analysis, beauties decaying at distances as low as 40 # m are in conditions to be detected, even in the vertical exposure, in more than 50% of the cases.
30
20
10
6
12
18
24
0
6
12
18
m
rad
Fig. 6. Experimental distributions of the angle differences between two segments of the same track. 10 grains picked up at each position 300 #m from one another. VII. VISUAL DEVICES
660
G, Baroni~et al. / Automatic devices' resolution in nuclear emulsion
900
n.tr.
\ 400
LL \
L
\ r...L__~
lO0
200
300
400
.'~, ( m r a d )
Fig. 7. MONTECARLO SIMULATION. Angular distribution of beauties decaying in a single charged prong.
I n this report, we have s h o w n that the accuracy o b t a i n a b l e with these devices is completely satisfactory even w h e n c o m p a r e d with very sophisticated direct m e a s u r e m e n t s a n d that it is possible to envisage simplified time saving analysis m e t h o d s with a d e q u a t e precision, allowing the i m p r o v e m e n t of the detection of single p r o n g decays or of very short p a t h decays. Of course, with these simplified methods, we do not use all the i n f o r m a t i o n stored in the emulsions which r e m a i n s available, for m e a s u r e m e n t s of special events. W e t h a n k Mrs C. Cattaneo, M r F. Cortellessa and M r s M. Federici for their careful work with these new apparatus.
References [1] G. Baroni et al., Nucl. Instr. and Meth. 214 (1983) 381; K. Niu: Proc 10th Int. Conf. on High Energy Phys., Madison, "Wisconsin (1980); W. Heinrich et al., Nucl. Tracks 5 (1981) 406; S. Tentindo and G. Vanderaeghe, CERN Report EPCEQ-IR1 (1981). [2] G. Baroni et al., Nota Interna n. 787, Istituto di Fisica, Universith di Roma (1981). [3] J.H. Shwe, F.H. Smith and W.H. Barkas, Phys. Rev. 125 (1962) 1024; G. Baroni, S. Di Liberto, P. Ginobbi, S. Petrera and G. Romano, Lett. Nuovo Cimento 242 (1979) 45; S. Petrera and G. Romano: Nucl. Instr. and Meth. 174 (1980) 53. [4] M.K. Gaillard and L. Maiani, Carges Inst. (1979) LAPP TH-09 C.