An improved technique for track profile measurement in nuclear emulsions

N U C L E A R I N S T R U M E N T S AND METHODS 48 (I967) I 4 I - I 4 6 ; © N O R T H - H O L L A N D P U B L I S H I N G CO.

AN IMPROVED

TECHNIQUE

FOR TRACK PROFILE MEASUREMENT

IN NUCLEAR EMULSIONS*

J E HALL and D J ZAFFARANO Institute for Atomtc Research and Department of Physics, Iowa State Umverstty, Ames, Iowa, USA

Received 22 June 1966 A mlcrodens*tometer utlhzmg a signal averaging digital computer has been developed to dlscNmmate between the charges of parUcles producing short tracks m nuclear emulsions For each track the photoelectric profiles of ten track segments, each 5 0ttm long, were averaged and the result plotted, yielding a mean profile curve. Results of measurements made on tracks produced by known

parUcles of charges 1, 2 and 3 are presented The normahzed area of the profile curve was found to give the best charge separaUon. Measured photometric mean track width values were corrected for the depth of the tracks m the emulsmn The charge groups were completely separated by measurement of 50/~m of each track, reqmrmg less than 15 mm per track

1. Introduction T h e p h o t o m e t r i c m e a n t r a c k w i d t h a n d the a p p h c a tlon o f th~s quanUty to the d e t e r m i n a t i o n of the m a s s a n d charge o f a p a r t i c l e p r o d u c i n g a t r a c k In a nuclear e m u l s i o n have been studied since the early 1950's Extensive w o r k has been d o n e by y o n F n e s e n et a l m Lurid 1-9) There are certain h m i t a t l o n s to the m e t h o d o f p h o t o e l e c t r i c d e t e r m i n a t i o n o f the m e a n t r a c k width m its present state The m o s t c o m m o n a p p l i c a t i o n o f this m e t h o d is to m e a s u r e the mean t r a c k width of s m a l l segments o f a t r a c k a n d then to s u m a n t h m e U c a l l y the results for the whole track, necessitating m a n y t i m e consuming measurements. Existing techniques generally require a m e a s u r e m e n t o f at least 100 ttm o f t r a c k length in o r d e r to p r o d u c e r e h a b l e results K e n y o n ~°) was able to o b t a i n fairly g o o d d l s c n m m a h o n between g r o u p s o f t r a c k s p r o d u c e d by k n o w n ions o f charges 1, 2, a n d 3 by m e a s u r i n g 62 5/~m o f each t r a c k H o w e v e r , the t r a c k s m e a s u r e d by K e n y o n were all l o c a t e d at a p p r o x i m a t e l y the middle o f each e m u l s i o n P r o b l e m s involving d e p t h c o r r e c t i o n were t h e r e b y a v o i d e d These m e a s u r e m e n t s indicate t h a t with a d e q u a t e d e p t h c o r r e c t i o n it m a y be possible to identify confidently particles p r o d u c i n g t r a c k s less t h a n 100/tm l o n g by p h o t o m e t r i c m e a s u r e m e n t o f the t r a c k profile W i t h the g r o w i n g interest in the analysis o f h y p e r f r a g m e n t tracks, s o m e r a p i d a n d r e h a b l e means o f charge d i s c r i m i n a t i o n is essentml Previous m e t h o d s r e q m r e the c a l c u l a t i o n o f t h e m e a n t r a c k width, either f r o m a g r a p h or an oscilloscope screen, after each sweep o f the i m a g e o f the t r a c k across the face o f the p h o t o m u l u p h e r t u b e I n the present investigation, e q m p m e n t was uUhzed to allow t h e storage a n d a v e r a g m g o f the m e a n t r a c k width curves for m d l w d u a l segments a l o n g a t r a c k T h e

m e t h o d allowed a large n u m b e r o f m e a s u r e m e n t s to be electronically stored a n d a v e r a g e d for a single segment o f the t r a c k Thus, after a c o m p l e t e t r a c k was scanned by the p h o t o m u l t l p h e r tube, a single m e a n t r a c k width value was calculated, which was the average m e a n t r a c k width for the whole track.

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PhotomulUpher Adjustable sht Rotatmg glass cubold x 25 eyepiece Eyepiece for wsual inspection × 100 objective Emulsion × 32 objectwe (reverted) Mirror Field stops F,lter Light source

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Fig. 1 Optical details of the mlcrodensltometer 141

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J. E. HALL AND D J, ZAFFARANO

2. Description of apparatus The optical details of the system utlhzed in the present investigation are shown in fig 1 The system was budt into an available Brower scattering microscope Ilium,nation of the emulsion plate was accomphshed by the

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F~g 2 Electrical detads of the mlcrodensltometer use o f a GE-855K 4 V, 2.5 A bulb with a straight wire filament A Wratten green filter (no 61) was placed between the light source and the emulsion plate to minimize effects of chromatic aberration m the optical system This filter was chosen because its maximum transmission was in the range 5000 to 5600 A It roughly matched the illumination wavelength to the m a x i m u m spectral sensitivity of the photomultlpher, which was an E M I 9524B with a lime soda glass endwindow photo cathode of the S-11 type. In an effort to reduce the amount of hght scattered in the emulsion, two field stops were inserted between the filter and the mirror which reflected the light beam through an angle of 90 ° The m~rror was hinged at the top and by means of a fine adjusting screw was used to position precisely the band of hght on the track Light reflected from the m, rror entered an inverted 32 x objecUve which reduced the size of the light band to 2 5 #m an the plane of the emulsion The vertical position of tins objective was variable so that the hght

band could be focused at any desired depth in the emulsion The track was viewed through interchangeable objectives, w,th a 100x magnification being used for measuring The image could alternately be observed wsually or electronically by operating a lever on the Leltz "combination binocular observation and straight photographic tube FS45" The photometer head was connected to the microscope body by a brass adapter, which contained a 25 x eyepiece for further magn,ficatlon. Thus, the image of the track had undergone a magnification of 2500 when it reached the photometer head Positioned dwectly over the optical axis of the 25 x eyepiece was a 1 0 cm x 1 0 cm x 5 0 cm glass cubold mounted on the shaft of a synchronous motor Above and parallel to the cuboid an adjustable sht was mounted on a moveable support A slit width of 12.50mm by 0 3 0 r a m was used, corresponding to 5 0 p m by 0 12 #m m the plane of the emulsion. The photomultlpher tube was attached to the top of the sht so that light passing through the slit fell on the photo cathode surface As the cubold rotated, the image of the track swept across the sht Two opposite faces of the cubold were blackened so that there were two sweeps (in the same sense) per revolution All of the photometer components except the synchronous motor were mounted in a hght-tlght aluminum box The motor was mounted outside the box to insure good heat dissipation The electrical detads of the system are shown in fig 2. A stabilized current supply was used to provide the power for the hght source. Using a highly regulated power supply, a negative I000 V was apphed to the photomultipher tube The signal from the photomulhpher was amplified by a transistor amphfier with gain variable between one and ten; the gain was always adjusted to provide an output signal of 0 35 V peak-topeak The signal from the photomultipher was noisy but fluctuated about some mean value This signal was put Into a signal averaging digital computer, Nuclear Data Model ND-800 Enhancetron-1024 and at the same time displayed on an oscilloscope Connecting the four-pole synchronous motor to a voltage hne of frequency 60 cps produced 1800 r p m and, hence, 3600 sweeps/mm (This is the fastest rate at which the Enhancetron can accept input m f o r m a tlon ) Due to our inability to achieve perfect alignment m the optical system, the signals from the two possible orientations of the cubold differed shghtly m phase. Therefore, only 1 sweep/revolution could be used After the Enhancetron averaged and stored each sweep, it stopped and awaited a 10 V trigger pulse before starting

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T R A C K P R O F I L E M E A S U R E M E N T IN N U C L E A R E M U L S I O N S

same power hne as the synchronous motor so that even if a variation in the hne frequency occurred, the relationship between the cubold orientation and the trigger pulse timing was always constant When the desired measurements on a track had been made, the contents of the memory of the Enhancetron were viewed on an oscilloscope and plotted by an X - Y pen recorder

AVERAGED TRACK PROFILES ---i

LITHIUM HELIUM HYDROGEN

3. Track measurement

In order to estimate the value of the mlcrodensitometer in determining the charge of the particle produclng an emulsion track, a group of 46 tracks was measured These tracks were located m a stack of Ilford K5 glass-backed emulsion plates exposed to a 2 3 GeV/c beam of K - mesons at the Alternating Gradient Synchrotron, Brookhaven National Laboratory The emulsion plates were 6" x 4" and were 600 pm thick before development The scanning and measuring of these plates was done by the Ames Laboratory Nuclear Emulsion Group The 46 tracks which were measured had been identified through kinematical

F~g 3 Typical averaged track profile curves

through ItS averaging cycle again It was necessary to construct a timing system which produced the necessary trigger pulse at the rate of 1800/mln As a result, only every other input signal was analyzed by the Enhancetron The timing system was connected to the I

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Fig 5 Measured mean track wzdth (MTW) values of portions of a hehum track at different depths in the emulsion, each point represents the measurement of the length of track traversing a depth of 4 0/~m analysts, and consisted of hyperfragments of Z = 1 and Z = 2 decaying through mesonic decay and 8LI " h a m m e r " tracks The number of tracks measured was relatively small because of the following restrictions placed on the tracks 1 at least 60 pm long, 2 dip angle less than 4Y an the undeveloped emulsions, 3 did not suffer large scattering Tracks to be measured were located and aligned parallel to the slit by viewing them through the microscope using a 10x objective After the track was positioned, the 10x objective was removed and replaced by a 100 x oll immersion objective The image of the straight wire filament of the hght source was focused m a 2 5 #m-wide band in the plane of the track, with the track positioned in the center of this band While viewing the photomultlpher output signal on an oscilloscope, the focusing and illumination were adlusted to produce an optimized symmetric pattern Measurement was begun on each track at a distance of 1 0 0 p m before the particle came to rest in the emulsion, since the tracks of all pamcles are very slmalar closer to the end Ten successive 5 0 pm segments were

measured on each track Each segment was measured for one minute, corresponding to 1800 sweeps being averaged and stored Thus, for each track 50 pm were measured, and the resulting curve represented the average of 18000 sweeps The curve was plotted by an X- Y pen recorder and the value for the mean track width of the track was calculated from this curve using the relation MTW = h w / H , where h is the height of the profile peak, i, is the width of this peak at height ½h and H is the average height of the background peaks Fig 3 shows the superposition of photoelectric track profile curves obtained for typical hydrogen, hehum and lithium tracks Each of the selected tracks was measured three times and an average mean track width was computed for each track The average mean track width for each track xs shown in fig 4, plotted as a function of the dlp angle of the track in the undeveloped emulsion. It was found that the charge groups were not as well separated If the measured half-width (w) of the profile curve of each track was plotted as a function of the dip angle

PROFILE

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EMULSIONS

sary Thxs was ascribed to the fact that the band of light was kept very narrow T w o least-mean-square strmght hnes were drawn through the measured values as shown Correction factors for each depth were obtained from these strmght lines and then the measured mean track width of each track was depth corrected The corrected mean track width values are shown in fig 6 The dashed curves represent the secant of the d~p angle, which gwes the expected angular dependence of the mean track width, since the number of developed grains observed per unit projected length increases with dip angle In order to get an estimate of the separation of the charge groups, the points of each group were fitted by least-mean-squares method to polynomials o f order one through five The second order polynomial was found to give the best fit The best fit curves were found to be separated as follows Between the Z = 1 and Z = 2 curves the separaUon was 3 0 root-mean-square deviations, between the Z = 2 and Z = 3 curves the separation was 4 0 root-mean-square devlaUons The three measurements made on each track can be used to estimate the repeatability of the measuring

Also, these measurements d~d not indicate that halfwidths are independent of the depth o f the track m the emulsion as proposed by some investigators 11). The opacity of a track decreases as its depth in the emulsion increases due to the scattering of hght which is projected upward through the bottom o f the emulsion plate, Into the image of the track In an effort to correct the measured mean track width values for the depth of the tracks m the emulsion, the following measurement was made A helium track about 5000/~m long was selected Th~s track, w~th dip angle of 6 °, had constant ~onlzatlon from a depth of 131 Itm m one emulsion plate to a depth of 173/~m m the emulsion plate directly above it Ideally a track traversing a single emulsion plate with low dip angle and constant iomzatlon should be used, but such a track was not found m the group o f plates examined in this research Because of the small dip angle and umform lomzatlon of this track, the mean track width values of different portions of the track were considered to be dependent only on the depth The results of these measurements are shown an fig 5 Thus, except for tracks located within 20/~m of the surface, very httle depth correcUon was necesi

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146

J E HALL AND D J ZAFFARANO

system. T h e average deviation for each o f the g r o u p s was the following LI, 2 0 % ; He, 2 2°0; H, 1 9°0 A n effort has been m a d e to identify possible sources o f error m the m e a s u r e m e n t s To check the varmtaon m the m e a s u r m g system, a 1 5 # m - w i d e b l a c k line on a glass plate was m e a s u r e d every d a y over a 45-day p e r i o d T h e s t a n d a r d d e v i a t i o n in these m e a s u r e m e n t s was 3 1%. I t as believed t h a t a large p a r t o f th~s e r r o r lies an the calculatlon o f the m e a n t r a c k wadth f r o m the profile g r a p h A digital r e a d o u t unit for the E n h a n c e t r o n as n o w available U s i n g this, the averaged profile curve f r o m the E n h a n c e t r o n could be o b t a i n e d m digital f o r m a n d the a r e a u n d e r the curve could be calculated b y a c o m p u t e r The use o f a s~gnal averaging d~gltal c o m p u t e r to store a n d average the profile curves has allowed m o r e r a p i d a n d accurate m e a s u r e m e n t s to be m a d e t h a n were previously possable Using this system, it has been possable to m e a s u r e the m e a n t r a c k wadth o f 50/~m o f t r a c k length m less t h a n 15 m m a n d t h e r e b y s e p a r a t e t r a c k s p r o d u c e d by p a m c l e s o f charge 1, 2, a n d 3 This macrodensitometer system has been used recently in

studying the m e s o m c decay o f heavy h y p e r f r a g m e n t s lz) a n d to s e p a r a t e t r a c k s o f Z = 1 a n d Z = 2 particles In a p h o t o n u c l e a r e x p e r i m e n t 13) T h e a u t h o r s wish to express thear appreciataon to D r Y W K a n g for the use o f e m u l s i o n plates cont a m i n g the adentlfied t r a c k s a n d for m a n y helpful discussions They would also h k e to t h a n k Mass M a r y A n n Pajc~c for her assistance m m a k i n g the n u m e r o u s mean t r a c k width m e a s u r e m e n t s References 1) S von Frlesen and K Knstxansson, Nature 166 (1950) 686 2) S von Fnesen and K Krlstlansson, Ark Fyk 4 (1952) 505. 3) K Knstlansson, Phd Mag 44 (1953) 268 4) B Waldeskog, Ark Fys 7 (1954) 475 5) K Krlstlansson, Ark Fys 8(1954)311 6) S von Frlesen, Ark Fys 8 (1954) 305 7) S von Frlesen and L Stlgmark, Ark Fys 8 (1954) 121 8) K Krlstlansson, Ark Fys 10 (1956) 447 9) T Johansson, Ark Fys 13 (1958) 129 10) I R Kenyon, Nucl lnstr and Meth 16 (1962) 348 lJ) M Ceccarelh and G T Zorn, Phil Mag 43 (1952) 356 12) y W Kang and D J Zaffarano, subm to Phys Rev 13) D G Scranton and D J Zaffarano, to be published