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An automatic measuring system for particle tracks in plastic detectors
Nuclc,tr
02
FOR
In,,trtJmcnls
AN A .JTO~,IATIC M['ASURING
SYSTEM
P/~RTICLE
W . T ~ , A K O W S K 1 , B. S C H O F E R ,
J. D R I - U ' ] ' E . S S O N N ' I ' A G ,
aJltd Metl'~od,, i~J I)hy~,t ~. ~, Re>,ezr,.~h 2 2 5 (19~.~) 9L I(x) N~)tl|~- | I~!lland, A)lt~.lcrd:lnt
TRACKS
IN PLASTIC
C, B R E C H T M A N ' N .
DI£'IT.CI'ORS J. BE.E~,
1-I. D : E C I ~ S E I - a n d W . I ' I E I N R I C H IJ~i~er
T ~[ 3'tv.'Sen, Physics Department, P.O. Box 101240. D - 5 ~ O Si,.::~n, Fi, d. Rt,p. German)'
Receiv :1 6 I:'.'~ruary 1984
In [ ~e la~,! .e~" _'years p i c t t t r e a n a l y s i s meIho.d~, h a v e ¢v,.)lved iltl(., a I.)c,,,verfuI t , : c h ) l i q u e [ o r illl~a))tll'~'nl,-" 11'~ i~J t~uc.t-;,r Ir:tck:., iI~ |)[.i.,llt'
dclectt ;. Wv !~avc develop~l a micropr,~esso)-based picture an'lalysi.,,s~ .,,tt'm for .uuton:alic track mcasu.'cmenls. 'I [..-"vide() psctures of partiel, tracl . seer, through a microscope are digitized in real time an.I the picture a~nalysis i'~ done by software:. The mic)os.c()p~ is equipp.*l wil~, a sta~e drive~a by stepping motors, which are controlle( by a ~ parate unit:roproce.~or. A PI)P ! ' /23 supervises the operate ~n or :)ql microproce,;:.ors and stores the mea,,~urc.dd a l a o n it~ ma ,.) stora~,e devices. "l'hi,, paper de,,cribes II': ~ard- and ~.)ftv,arc of the .?-ste~) and the metla(_,ds of pit:lure analy_,,is which are u~a~d for t')e track identification and mea~uremen~l
1. |nlu-)duel i.)n Stt.,agl3 ionizing particles f o r m latent tracks of damal:: d t)):a~crial in nuclear track d e t e c t o r s like plastic foils c' gla,,,es. By etching of the detectors the surface is remo',,:':l v.~th a c o n s t a n t el;:hing velocity o,. li the dauna[ ~ ex,.eeds a threshold the m a t e n a l is etched a w a y a l o n g ~he r a i e c t o ~ " of .'.Ire pa,'ticle wtth a track etch vel{x::i'L, o, - ~:~.T h u s an ,;tch c,--ae is formed, which c a n be oh:, rve,! a n d m,'asttrt:d v,i!h a microscope. F o r c o n stant ;:chi~)g c o n d i t i o n s the length of this etch c o n e intreat ..:s '~,;tl" increasin~ radi:,tion d a m a g e , i.e. increasing ~.n .:rgy loss of the particle [1]. In : ty,)ical experiment u~i_ag plastic nuclear track detect, J.-; .::, stack of thin p~ ~.,tic foils (each of s o m e hund).':l /.:m thickness) i, eq)o,.:ed to a n a c c e l e r a t o r b e a m -:.f h:'avy ions or to '.!,c c.~smic radiation. C h a r g e s a n d n ~'ss~:- of heavy nuclei car' be d e t e r m i n e d f r o m the evzcrg.? Ins: as a function of their range. F o r this p u r p o ~ the le:,.,Fh.~ of the etch cones have to be m e a s u r e d . This was t:.:,ne ~n the past ~y h a n d . m e a s u r e m e n t s of the thre,6-,':,m,:rtsional co,ordinate,*, uf the etch cones. T h , use of p i c t u r e :maly:;i', systems for a u t o m a t i c m e a s c c m , : n t s of heav2¢ ion t<'acks in plastic n u c l e a r track tetcctors w a s rec~n~.l:.,' ,'Jescribed b y different ~uthol [;.-8]. T r a c k ntcasurc:ntents with video .'yslems using .,.ethods of picture an.'.)ly~L!sc a n be p e r f o u m c d if the n'.asLJrement is restricted t:~ a focal plane o n the surfac:, o1" the detcctor. F o r ~ ~,- ;topic by direct illumination t ; e l d p s e s which are fo aed b y the i n t e r s e t l i o n of the et .'.a c,)nes ',,.ith tl~e surf; :~: of the plastic material, appez, as d a r k areas o n a I::i~.hter b a c k g r o u n d . 'l'heir positi,:3s, areas, perimeters at:d o t h e r geometric qua,aLlties c.:n !re m e a s u r e d using "icture analysis systems_ 0167-. ')8"i/84/503.f~0 C Elsevi:.r Science Publishers B.V. (Nort -lt,-,iland Physics Publi:;i~ing D~vision)
l r o m the a r e a of the ellip~'s on the d e t e c t o r s u r f a c e the e n e r g y loss c a n be c a l c u l a t r d . [tov,~ever, these a u t o m a t i c ) a e a s u r e m e n t s are less precise if the etch figures *~e f~awed by a s u r f a c e s t r u c t u r e on the plastic foil_ O n the c her h a n d the use of c o m p u t e r i z e d i m a g e analysis t: ables the a p p l i c a t i o n of m~clear , r a c k d e t e c t o r s in (: p , ' r i r r e n t s collecting large u a n t i l i c s of d a t a , which '. :re in-possible until now. S t a n d a r d i m a g e analysis system,~, w h i c h are widely .ed in bio!ogy, medicine, r e m o t e sensing a n d material icnce, are c o m m e r c i a l l y available h)r c o m p u t e r i z e d , lea.,,uo~mcnts of n u c l e a r tracks, but they are rather ,:t),..nsive a n d unf'~exible. W c built :t p o w e r f u l systcna ' : sed on a real time video digitizer a n d a 16-bit micro;ocessor. T h e p i c t u r e analysis is d o n e b y c o m p u t e r . r o g r a m s that c a n be a d a p t e d to various ~pecial p r o b ~IIlS.
"l'l'~ s3,stent for a u t o ' 3 a t i c t r e k m~z).stluron~nts T h e a u t o m a t i c m e a s u r i n g device is s h o w n scl-.ematit a l l y in fig. 1. 'The plastic d e t e c t o r s are mounte.'l o n a micro_w,..ope stage, which c a n be m o v e d over 10 c m :,< 10 c m b y s t e p p i n g m o t o r s with an i n c r e m e n t a l step size of 0.25 /~m. A n a d d i t i o n a l s t e p p i n g m o t o r moves ~he fine a d j u s t m e n t of the micr~_)~ope fo(.'us. A c o n t r o l device allows ntotion of the stage u n d e r m a n u a l or c o m p u t e r control. 'I he particle track.-; seen t h r o u g h the m i c r o ~ o p e b y a video c a m e r a are identified a n d m e a s u r e d b y a picture analysis system. F o r this p u r p o s e the p i c t u r e is digiLizcd b y a video a d a p t o r a n d the d a t a are t r a n s f e r r e d to o n e of the p i c t u r e analysis c o m p u t e r s . "[he picture analysis
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a n d mea:'ured tracks c~n the foil frolla the cf>ortl61atcs of the respective field of view. T h u s the activities for the anlonla|~c track m e a s u r e m e n t are s h a l e d between different c o m p u t e r s which work parallel on differcnl tasks. T h i s m a k e s the system fast.
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T h e m o v e m e n t of the m i c r o s c o p e stage i~ controlled b y an B-bit m i c r o p r o c e s s e r M C 6[~02. Fig. 2 shows how this m i c r o c o m p u t e r is interfaced to different ,-'ompu.,ler~ts of the systenL A m o n i t o r a n d the software for ~,ta£'.e m o v e m e n t a n d c o n t r o l are stored in 8 khyte R O M . 2 kbyte R A M are available for d y n a m i c storage of flat;, a n d for special p r o g r a m s . "riffs c o m p u t e r is lioked to th,.: PEal:' via two serial intt:rfaces M C 6850 (ACIA}. These are tLsed for transfer of c o m m a n d s a n d d a t a or for d o w n l o a d i n g p r o g r a m s in m a c h i n e code w h i c h were gener,:ted in the P D P using a cross as,;embier. Six p e r i p h e r a l interface a d a p t o r s M(" 6~r20 (['IA) ar,.: used to COltnect the m i c r o p r o c e s s o r with tlh~ eb.'ctronic devices for stage m o v e m e n t . The x-)" Slelq.-'ing v.',,Hors are c o n t r o l l e d b y a unit which is available v:ith the m i c r o s c o p e stage. It c a n be used u n d e r c o m p u t e r control w h e n the pulses for the s t e p p i n g m o t o r s are. get,cra t e d b y software r u n n i n g on the M C 6802. By this the velocity of the stage m o v e m e n t cart easily t,e controlled a n d it is possible to define an acceleratio:l a n d a ciecele r a t i o n p h a s e for m o t i o n from point to point. "l his allows to move the stage with hJgh velocity "wizhoul losing steps. A s e p a r a t e c o n t r o l logic was inst,.lied ft~t the z-coordinate. In a d d i t i o n to the s t e p p i r ~ m o , o l s x~.e
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is d o n e by software. We u~,e four of these picture analysis c o m p u t e r s whicit c a n wock parallel o n different pictures T h e m a i n p r c ~ e s s o r o f the system is a P D P 1 1 / 2 3 It sV'rts o n e of the f o u r p i c t u r e a n ; l y s i s c o m p u t e r s w h i c h stores the picture in its m e m o r y . While the picture analysis c o m p u t e r is m e a s u r i n g the particle tracks, the P D P 1 ] / 2 3 t r a n s m i t s c o o r d i n a t e s of d i s p l a c e m e n t to the stage control, w h i c h then moves the stage to the next Eeld of view. A f t e r this the P D P s¢:lccts -', picture analysis c o m p u t e r w h i c h is idle a n d initializes it to take the next picture a n d to w o r k on it. Parallel to these activities the P D P has to collect the t r a c k d a t a detenrtined b y the different p i c t u r e analysis c o m p u t e r s a n d to calculate the a b s o l u t e co¢~rdinates of the detected
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; n s t a l l e l i w ' r e m e n t a l encoders. T h e i r signals arc fed mtc' co ruler.; that c a n be read by the nticroproccssor. This t o n b i n a t i o n of e n c o d e r s a n d s t e p p i n g m o t o r s provides a kigh reliability of stage m o v e m e n t . 1"he ~,:tage m o v e m e n t c a n al~) be p e r f o r m e d u n d e r nlanua! control. F o r tl'is p u r p o s e t h : x - y s t a g e c o n t r o l unit generates the pul~;es for the s t e p p i n g motors. "l-he frequency ol these pulses is defined by a joystick ~,'hich is installed o n the m a n u a l c o n t r o l b o a r d s h o w n schematically in fig. 3. The z - c o o r d i n a t e stepping, m o t o r c a n be m o v e d b y pressing o n e of the tv,-o b u t t o n s for z-movement. T h e m a n u a l control f a n c t i o n of the stage m o v e m e n t is n e c e s s a r y for setting up the system a n d for definition of a c o o r d i n a t e systern on the d e t e c t o r foil,;.
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Since we use f o u r picture analysis c o m p u t e r s in con, ninati0n with o n e video system, we need the video a d a p t o r shown in fig. 4 to link the TV system to the picture anal;~.,is c o m p u t e r s . In oL'r video system we us~a H a m a m a t s u C 10130 TV career ~ with a Saticon T u b e . which h.',s only a small lag of 2%. P a r t of this s y t e m is a c a m e r a control unit, which allo ,'s the c o r r e c t i o n of a s h a d i n g of £ae video picture. A ditionally it generates addresses to store the digitized picture into the c o m puter mentory. The.~e 18-bit a( Ir,".'~cs a n d Ihe video signal are fed into the video ada: tot, A T R W ;.0(J7 A D C digitizes -~ e video p i c t u r e in real nine within 40 ms. F o r 512 × 51. pic ~;e points (pixels) the grey lev,.:ls are d e t e r m i n e d v th :: reso,ution of 256 steps a n d sq,red in o n e byte. 'I le i , t f o r m a t i o n of two bytes is colh:cted in a late} an I th.~.,t transferred via D M A to the p c ' u r e an.'dys:s c~ mputer. Based on the 18-bit addre.,ses s u p p o r t e d b y t e c a m e r a control unit 24-bit addre, ses for the 256 kb.~ :e picture m c m o r y are generated v4thin the video ad plot. T o display the c o n t e n t s of ~he picture mcn-ory .:,n a video m o n i t o r "1 D A C is used. T h e video picture .; synchroni:~ed b y the address pulses generated b y the m c r a control u n h . The data bus a n d the 24-bit a ..:lre.ss bus of the video a d a p t o r can he switched be~we¢ I n s t p i c t u r e ;malysb, c o m p u t e r s . A switch-over i'.; a~ v a t : d by the p i c t u r e
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analysis processor.,, thenl~,elue~. A dur "rely I',:a,:l fl'lflll the highest a~dress activates th--' A D C • nd D M A logic a n d the d a t a of a digitized p i c t u r e are t r a n s f e r r e d to the p i c t u r e m e m o r y of the processor. T o start the d i s p l a y m o d e of the s y s t e m it is necessary to write a c o n s t a n t N to the highest a d d r e s s of the processor. This c o n s t a n t is stored in a register of the video a d a p t o r a n d initializes it to d i s p l a y the p i c t u r e m e m o r y for a time of N × 40 ms. W h e n the video a d a p t o r is linked to one of the f o u r p i c t u r e pr.x:es,~ors all rcquc,;ts of the other three pro~essors are ignored. 2.3. 27le picture analy.~is ( o m p u t c r v P'ip.. 5 ,,,Imws ,,thematically the: p i c t u r e anMysis c(mtpurer. T h e C P U is a ],"}-bit n t i c r o p r o c .sor M C 6~ly')O. Jt is s u p p o r t e d with 256 k b y t e p i c t u r e m e m o r y a n d the s a m e a m o u n t of p r o g r a m a n d d a t a m e m o r y . A d d i t i o n ally 24 k b y t e of R O M a n d 4 k b y t e of R A M are available for a residem m o n i t o r a n d :;tack o p e r a t i o n s . T h e a d d r e s s b u s a n d the d a t a bus of this c o m p u t e r can direclly be c o n n e c t e d to the video a d a p t o r as tleserihe(t above, T h e p r o g r a m c o d e for picture .',nalysis is generated by i,, cross a s s e m b l e r in the P D P . It c a n be d o w n l o a d e d into the p i c t u r e analysis c o m p u t e r vm a serial link. D u r i n g the p i c t u r e analysis all d a t a e x c h a n g e d between the M C 68000 a n d the P D P are t r a n s f e r r e d o n a parallel line. T h e pictnre analysis c o m p u t e r c a n be c~,nnected wi'_h the m a n u a l c o n t r o l b o a r d (fig. t). "lhi,, all.wn Io display a c u ] s o r in the picture rnt:nlc,ry which c a n be m o v e d by b u t t o n s o n the m a n u a l c o n t l o l b o a r d . T h e c u r s o r is g e n e r a t e d b y a special ~:,ftware w h i c h is initialized b y a c o m m a n d f r o m the P D P . A f t e r a take d a t a c o m m a n d f r o m the curs.or b o a r d the grey level o r the p i c t u r e p o i n t m a r k e d by the c u r s o r are d e t e r m i n e d a n d transferred to the P D P . This possibility is used w h e n
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with x ' r the n u m b e r of first picture point of segmen: a. x I the n u m b e r of last picture point of segment *, ,,1 the n u m b e r of segments. T h e centre of mass c o o r d i n a t e s are given by:
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with y ' the line n u m b e r of segment ~. T h e perimeter of :t track can be m e a s u r e d I.l:¢ sumt h i n g u p all peripheral points in units of pictur~ points. W e define the perimeter of a detected object as follov,'.-.: II
P = 2 n + l i + l , , + ~.[I,-I,_tl. p a r a m e t e r s for the p i c t u r e analysis p r o g r a m s have to bc set up for a nev.' type of d e t e c t o r foil.
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3.2. Sepuration of background 3.1. Stctn,'lurdprocedure for circular track; T h e particle track s h o w n in rig. 6a will b c used as an e.;anlple to explain tile s o f t w a r e for Irltek m e a s u r e m e n t . F o r this p u r p o s e the track win- analyzed o n l y with low n l a g n i f i c a l i o n so tllal the p i c t u r e analysis system sees an t)bjc.ct with only a "c~ picture points. Fig. 6b sho',~.s the section of d a t a c o r r e s p o n d i n g to this track, which are stored in the m e m o r y of the picture analysis system a f t e r digitizing_ T h e n u m b e r s give the grey levels of the p i c t u r e points_ Z e r o is black, 255 is white. If we define in this case a thresh~,ld value of 40 in grey level for particle tracks, the a r e a within the b o u n d : ries of the solid line s h o w n in fig. 6b is a c c e p t e d as a t:ack. T h e p r o c e d u r e of our p a t t e r n r e c o g n i t i o n p r o g r a m for detection a n d analysis of tracks is to e x a m i n e in a first step the stored video picture line by li",e s e a r c h i n g for s e g m e n t s that c o m p l y with the grey leve~ c o n d i t i o n . In o u r c l a m p % in fig. 6b 13 ,~egntertts are detected. The next step is to c o n s t r u c t objects f r o m the detected .,,cgments. T w o s e g m e n t s in a d j a c e n t lines b e l o n g to tile ',:tu~e object if they o v e r l a p in their x - c o o r d i n a t e s . F o r s e g m e n t s that d o n o t b e l o n g to a n object that was a l r e a d y detected in lines a b o v e a nev,' object has to be defined. If one object that w a s a l r e a d y detected earlier has no c o r r e s p o n d i n g ~ g m e n t in the actual line this object is defined as detected completely. F o r the detected objects some g e o m e t r i c quantities are calculated.
A m o r e c o m p l i c a t e d situation arit~es for the two small objects s h o w n in fig. 6a. We see on the detector s u r f a c e objects which are not d a r k in the centre. Cbjects of this type in m a n y cases are surfact, structure~. But also particle tracks s h o w a reflection of lig.h: qll the central p a r t if the), originate from s t o p p i n g p:lrlif:h~s or f r o m par'Jcles incident with small angles to the ,!erector surface. T h e picture analysis p r o g r a m s h a n d , " these objects, w h e r e the central grey levels are not h,.!,.~'.,.' IIte t h r e s h o l d of detection, like completely black objects. T h i s is achieved by the p r o c e d u r e , that the white g a p between two segm" Its in a line belonging to I|ie same object is fillt:d. S:ace v,,e lose some informaticr', a b o u t the measured object in this v,'ay, we additionally calculate the s u m of grey level values for a central N .'--:V pixel matrix, where N can be freely a d a p t e d . In combin a t i o n with the size of the object we have a cri.'.erion to distinguish particle tracks from detector surf~',:e strutlure.
3.3. Overlapping tracks P r o b l e m s arise from overla~_ning tracks like those s h o w n in fig. 7_ If the overlap is not ttx'~ large, they c a n be identified based o n the ratio of area a n d perimeter. W - defined some ~ o r e geometric quantities to identify these n o n circular objects_ The lengths of the projections d , , d,. of a detected
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[hC C¢IoltJI11¢1[¢2 ,IKL'Y,. lilt.' cov.,rl,mcC Cquals /elO. ]hl.lt [hL'['l ;l~.;llll llte d f f [ c r c n c c oI" the ~l,.)jcc[[Ol]% ~ a hl~.I]ll-ic~lil[ I[]dlL';II.IL,II For a r l o n - c l r c u l a r OhjCL'l l ~ o o v e r l a p p i n g c i r c u l a r track'. '-'an he .,ep.tr,llcd based o n an o p e r a t l o n called crohloil l-i~i, H'Ie,IIP, ,l ~ ] l n n k m g o f the COIIIOLIr o f an ~bject OlllllLIlt~ ~l)l lIL'n p h c r a l points. By ~,LIi.'Ce~Mve cr'.~sion o f Lwt, ~werlapp i n g tracks t w o s m a l l e r separated objects arc Im~duccd ] heir u'enlru o f ma.s~ t_-oordmat.cs arc Ihc c o o r d i n a t e s o f the [['ack~ Kl3~/~ll]g tlte degree o f clt)~,io[) thcd ,lrc~it~ O[ the e r o d e d objecl,s can hc corrected to get Lhe o r i g l n . d Iritck areas, l}a~ed on the ',echmque o f Seglll,~l}[:ilio,'1 ,oI d~c objects i n t o line :,cgmcn:s. w l u c h ~,: u:,e. ~' i~ po'.~iblc I~) d o l]t¢ ero.~lOn w l t l i ~larldard ¢}perator~ ;ix defmL'd h'~ ~(t)mlg el al. I9]. 3 4
K [ h p m ~1 tr~l~ t
"]-ho particle Irilck~ %ho~.Lll ill ~1~ () LIIId 7 ~trc p l o duLed b?,. particla.,, perlt?lr,d]i~ the d e t e c t o r I ; e r p e n d ~ : w tar to its ~,url';,~:¢. T h e r e f o r e dle IIH,2r',ccHOI~ O[ I]Ic :2[~}i COliC i~.llh the d e t e c t o r ~tlrI'iLce Is a CIRIC ]'OT rlofl-XCrllcttl Irajc:c[orlcs thl% etch fJo,ure 1~ ,m clhp~c I,'.~' tile oiIc s h o w n llt fig. X. For thc'~c Irack~ the length ,::vJ ,~tv:atatlon of m i n o r a n d m a j o r axc~ p r o ; t d c tnlt.~rll~dl:O!l a b o u t the d i r e c t i o n of ~.he tra3ector', and the encrgx, !o~. o f the p a r n c l e W e c a n a n a l y z e elhpt]cal object:, b'. d e t e r n u m n g a d d m o n a l l y to the c o v a r i a n o . ' also the v.'natacc., l'lg 7 ".ltcropnotograph of t~o overlapping track~. I J-I
lqg ~1. Eihplical track and definmon of parameLer~.
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~2
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4.2. 1:-leaning point software for the M C 68000
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4. Soft~itre
F o r s o m e o p e l ' a l t o n : of the p i c t u r e analysis p r o g r a m s it is n , x e s s a r y to use floating p o i n t Operations. F o r this p u r p o ~ we d e v e l o p e d a floating p o i n t s o f t w a r e p a c k a g e for the M C 6 g O ~ wigh a p r e s e n t a t i o n of real a n d d o u b l e precision integer n u m b e r s b e i n g identical to the P D P 11 s t a n d a r d . T h a t m e a n s that all results of the p i c t u r e analysis c a n be directly used b y the P D P w i t h o u t f u r t h e r c o n v e r s i o n s . All floating fmint o p e r a t i o n s un the M ( ' 68000 arc a d i v a l e d hy line It)Ill e m u l a t o r Irap',. F o r 32-bit f l o a t i n g [mint variables in h i d d e n bit m.)rmalization a d d i t i o n a l l y to the f o u r basic :'rithmetic ~)pcrations the f o l l o w i n g o p e r a t i o n s are a,,;,.lable: c o m p a r e , negate, integer to floating p o i n t co,l,'ersion a n d vice versa. F o r 32-bit integer variables n m h i p l i c a t i o n a n d divisinn are defined. A d d i t i o n a l l y the s q u a r e r(×~l ftmctions for 32-bit integer a n d floating ~ i n t o p c r a u d s are available.
4.1. System sc4iware for the M C 6~ 'g'K) 4. 3. PActure anal|'si3 software F o r the pic,ure analysis comF_,ter we developed a special m o n i t o r c o n t a i n i n g system routines w h i c h simplif_y the o p e r a t i o n of this system u n d e r the c o n t r o l of the P D P 1 1 / 2 3 . T h e m o n i t o r c o n t a i n s some b a s c f u n c t i o n s like stalling a n d tracing p r o g r a m s rur, nin~ in user m o d e o r in supervisor mode, m e m o r y change: for byte, w o r d a n d Iongv,'t)rd and r u n n i n g a m,-.'mot3' t;::d p r o g r a m . A d o w n load function ( a n be used to ; , a n s f e r p r o g r a m s ira m a c h i n e (:ode ~enerut-.d b y a c r e l s a s s e m b l e r on the P D P into the m ; c r o p r o c c s s o r mern:}rv. F o r this p u r p o s e the M C 61~0<~'Jactivates a transfer ? r o g r a m on the P D P . Files cot:taining s-records c a n thug b e l o a d e d f r o m a n y ntass storage device of the P D P . ] ' h e addresses in the miclopro+.cssor m~.mory c a n be the ~ m e as defined in the .',-records or shifted by a c o n s t a n t , or c.m be c o n t i g u o as f r o m an a m b i g u o u s starting a.(dress. T o facilitate alter-native wor',: o n the P O P a n d the M C 68000 a m o n i t o r tune;ion ft-.r t r a n s p a r e n t terminal m o d e is available on the m i c r o p r o c e s s o r . T h i s allows the user of the m i c r o p r o c e s s o r terminal to w o r k w~th the P D P like on a terminal directly r a k e d to this m a c h i n e . A further g r o u p of m o n i t o r ft.=:tions enables tests of the system as a picture analysis ~,,'stem. These are basic functions for digitizing a picture or d i s p l a y i n g "he content of the picture nremor2," ~n .= T V m o n i t o r . F o r test purlx)ses dif[erem p a t t e r n s cat~ be s y m h e t l z e d in the memory. A f t e r a p r o c e s s o r reset the -nonitor is w a i t i n g for i n p u t from the P D P c,r f r o m the- terminal, d e f i n i n g the s l a t t i n g address of a program_ ']'he p r o c e s s o r reset c a n
T h e p i c t u r e a n a l ) s i s s o f t w a r e r u n n i n g on the M C 68000 follows a n idea developed b y H c n k e [2]. It consists of two b a d e p r o g r a m s , ] h e r o u t i n e " C O L U M N " d e t e r m i n e s detected line s e g m e n t s h a v i n g grey leveh. below or a b o v e a t h r e s h o l d a n d stores these d a t a into m e m o r y . T h e routine " F E A T U R E " r e c o n s t r u c t s detected objects b a s e d o n the d a t a p r o v i d e d b y " C O L U M N " . " I ' E A T U R E " calculates all the track d a t a de.scribed above. The ':ariable!; d e t e r m i n e d hy " C O I . U M N " a n d " F E A T I Y R E " are stored in a d y n a m i c r : x o r d s t r u c t u r e . T h e o v e r w h e l m i n g p a r t of the 256 k b y t e p r o g r a m m e m o r y is used for this p u r p o s e . T o analyze a p i c t u r e the P D P activates " C O I . U M N " to r u n or, rne M C 68000_ After c o m p l e t i o n of this p r o g r a m the P D P roceives s o m e s t a t u s a n d error c o d e s f r o m the M C 6tIIKIO a n d then starts the p r o g r a m "FI+]A TURi:."_ " F E A T U R E " also transrruts status c o d e s a n d the results of the p i c t u r e analysis. T h r e e different versions of " ' C O L U M N " are available. T h e fastest version is r u n n i n g with a c o n s t a n t dete~tion t h r e s h o l d of grey level. A s h a d i n g of the p i c t u r e w h i c h is not completely c o r r e c t e d by the video c a m e r a c o n t r o l unit c a n be c o n s i d e r e d hy versions of " C O L U M N " with n o n - c o n s t a n t det,mution tllreshulds. F o r a s h a d i n g that is c o n s t a n t for all sucoessive pictures the thresholds are defined b y a s h a d i n g matrix, H e r e the t h r e s h o l d s are given b y c o n s t a n t s for fields of g x 8 picture points. In the case that the s h a d i n g change~. f r o m o n e p i c t u r e to the next o n e a d y n a m i c threshold c a n be used. In this case a n individual threshold is
II/ "lrako, ~1~j el tJl. / A ut,)~nanc rpJeasurtptl~ r)'stem fi~r particle tracks c:~lculatcd f . r each sitlet~ lira: in the following way. The I:,, ,11 rlt:lxima of grey levels in the video line are linked togcthcJ J,v a p o l y g o n line. "[heft this is shifted b y a et:nstal-,t ~AI.se, I,~ f o r m the threshold for d e t e c t i o n of d a r k objects. T h e execution of the picture analysis by the p r o g r a m version with c o n s t a n t thrc:;hold i'~ p e r f o r m e d in 0 . 7 - 0 . 9 s. d e p e n d i n g o n the contelit of tht.: Fisture. The version with the s h a d i n g m a t r i x is a little ~kJwcr. it lakes a b o u t 1 2 s, w h e r e a s the p i c l u r e analysis using a d y n a n l i c threshcfld takes a b o u t 5 s p e r picture. All p i c t u r e a n a l y s i s p r o g r a m s were written in the as,~embler langu:~gc of the M C 6 8 0 ~ ) attd , r a n s l a t c d to alat,lline code USiltg a cross a s s e m b l e r r u n n i n g on tile PI31'. ¢.4. S o f t w a r e r u , m m g
on the P D P
T h e p r o g r a m s rnnrting o n the P D P use the hasic f u n c t i o n s of the p i c t u r e analysis s_vstcli~s a n d the stage corArol p r o c e s s o r for two m a i n p u r p o s e s . 1 testing the a u t o m a t i c m e a s u r i n g device a n d setting up p a r a m e t e r s for a special a p p l i c a t i o n , 2. p e r f o r m i n g a m e a s u r e m e n t for tracks on a d e t e c t o r foil. "I'o define the p a r a m e t e r s for a m e a s u r e m e n t d~ffere a t tests c a n be p e r f o r m e d . A c u r s o r c a n be m o v e d on the video screen a n d the grey levels at these positions c a n be read by the PDP. A d d i t i o n a l l y the grey levels a l o n g a video line c a n be d i s p l a y e d o n the screen as a d i a g r a m . Based o n this i n f o r l n a t i o n o n e of the three •,ersions of the p r o g r a m " C O L U M N " c a n be selected. To set u p the s h a d i n g matrix, a p i c ; u r e which is def¢c,:ttsed or is c o m p l e t e l y grey, with no i n f o r m a t i o n in it, c a n be digitized. r i t e ~hading m a t r i x d e t e r m i n e d in this w a y can be c h a n g e d b y a c o n s t a n t or b y the i n f o r m a t i o n of a n o t h e r picture w h i c h c o n t r i b u t e s with a ,,,,'eight f a c t o r betv,'een 0 a n d 1_ W h e n the detectior, threshold i,. defined, single piclures c a n be a n a l y z e J a n d detected objects c a n be m a r k e d by a w h i t e (or black) p e r i m e t e r a n d a central cross. T h e results of the track m e a s u r e m e n t s are p r i n t e d o n the ter,rfinal. T h e o p e r a t o r cart select tracks for w h i c h he believes the detectton to be problen'tatic a n d test the o p e r a t i o n o f the system fox these cases. A d d i t i o n a l activities to set up the system for meas u r e m e n t are the definition of the a r e a w h i c h hits to be s c a n n e d o n a plastic foil attd to m e a s u r e the m a g n i f i c a tion, that Ineans the size of one picture point in squ:tre |nicrtnls,. T h e measurennent of all tracks on a plastic d e t e c t o r foil is p e r f o r m e d b y m o v i n g flu m i c r o s c o p e s t a g e in m e a n d e r i n g path_ D u r i n g Ihis C r~eration the field of ,'Jew is sEiftcJ b y the size of the I old of view m i n u s an offset_ This offset is at least as lar ~- as the radins of the largest object which has to be m e a s u r e d . D u r i n g the
99
Iileastlrt.:nlent only objects with cc-ardinates inside a f r a m e of Llte size of this offset arc accepted. By this p r o c e d u r e we avoid to take d a t a oi tracks that are not c o m p l e t e l y inside the field of view. O n e m a j o r p r o b l e m is to contrail the focus of the m i c r o s c o p e , when a d e t e c t o r of larger size is SCalmed. F o r lc,wer magnifieatiol'l where this p r o b l e m is not as critical we d o this by a p p r o x i m a t i n g tl,c surface of the d e t e c t u r as :t plane. F o r this p u r p o s c the z-coordinates of three different points on the d e t e c t o r surface have to be m e a s u r e d d e f i n i n g the plane. D u r i n g the s c a n n i n g p r o c e d u r e the P U P calculates the a p p r o p r i a t e z-cc~_wdi~ate f r o m this plane. Thl,,, fast p r o c e d u r e c a n n o t be used for higher magr,,fication:, ,.,,here the focus has to be very pr-~cise. Here o l h e r method,, i~'.','e to he developed to define a focus f r o m the c o n t e n t of the picture. The sizes of the objects a> a function ol- Iht2 z - c o o r d i n a t e s h o w an e?.tlenlLinl at the p o i n t of focus. A n o t h e r q u a n t i t y for defining a focus is t_~le contrast c f the whole picture. T h e g r a d i e n t of the grey le',el values of a picture as a f u n c t i o n of ti'.c :'-co~wdinate I:as a p e a k at the focus position. U n f o r t u n a t e l y ~!au.,c p:-oced u r e s are rather slow and thus would reduce tile :,pc~'d of the m e a s u r e m e n t . The d e v e l o p m e n t of ai) alnofoctts for our system is u n d e r progress.
5. C o n c l u s i o n Tile m i c r o p r o c e s s o r based a u t o m a t i c picture an.d~',is s,:stem presented in this p a p e r i.,, a powerful toot f,~r m e a s u r i n g nuclear tracks in plast, ic detectors. The m:~l~r a d v a n t a g e of the system is its flexibility Since the picture analysis is p e r f o r m e d by software, we are a H e v.,~ a d a p t tile analysis m e t h o d s to special problems. The s y s t e r : is a p p l i c a b l e for a n a l y z i n g nuclear track detectors which ",,,'ere exposed to high particle fll:xes of accelerators. O n the other h a n d it c a n bc used also in c o s m i c ray experiments. A u t o m a t t c scantling is :ha adv~ n t a g c when dealing with low particle fluxes of cosrnic radiation_ Elliptical tracks of particles incident from various diieclions c a n be ffle~snred. W e have used this system in an experintent where 2 × 107 etch copes were m e a s u r e d to investigate the iftteraction m e a n free p.ilh of relativistic heavy iort b e a n i f r a g m e n t s [10,11]. F u r t h e r m o r e a special version of the zystem will be used in a cosmic ray e x p e r i m e n t [121 where it is necessary to analyze a b o u t 600 m-' of plastic track detectors. W.H_ t h a n k s E.V. Benton, R.M. Cassou a n d R.P. .-lenke for valuable discussions. This work is s u p p o r t e d b y the B u n d e s m i n i s t e r fur F o r s c h u n g u n d Technologic, No. 01QV3127. No. 01QV2032, N o 01QV3029 a n d No. 01QV2130.
1(~
If~ Tro~[ow; ~.'i et .L / .( ut<,-!',t1( mee.rurinf4 .;ysrcm f~r piirll( h Ivtl('k s
Referer,~s [I.] R.L. Flcb,chcr, P.B. 1~c¢ ard R.M, 'Walk¢r. Nu.:Icar tracks in solids ,Univcrsily of Ca!i[ornia Pre~, 1975~, [2] R.P. Her, k¢, E.V. i].cnt(:q znd R.M. Cassou. Solid slate nuclear Irack d,~,ec|ors .'Pcx[,ar:on, New York, 1980) p. 509, and private discussiox ': [3] E.X. Shirk and P.B. Prier 16~h Int. Cosmic ray Conf. Kyoto, Japan, vol. 11. (19;9j -'~. ['6. [4] J.H. Adams. Nucl. Tracks 4 [1980) 67. [S] W Trakowski, B. ~.~ch6fer anP W Heinrich. CERN rc'Tw,rt 8,,-07 (1981) p_ 44~. [6] J. Berr. J. Pipper and W. H¢.nri:h, J. Phys. E 15 (1982) 439. [7l W_ Hei~rich. W. Trakowski, , )icr.r and It. ,'~-huchl.. Pr~.'. ] l t h Int. Conf. on ~)lid sl, I~. nue'.ear tr;,ck de!.s=tor,,, Bristol (]9M) p. 9 l l .
[k I W Heinr~,.h. II. D-et.h~l, It. Rudat, I" V. l:knton ~,ml [). ['|iILtebrand Nu,c'L nslr. mnJ M¢th. l'.~J(19141) .169. [gJ I.T. Youna. ft.[.. Peverim. P.V,. 'Vcrh,eck and P J. ",'on Olterlu,o, C o m p . (..;"aph. ln~ds¢ Pr(~. 17 (19Rl) 1:4~. {It)} W. lleinnch, I']. Dr,cchsci, W. Tr*kow:,ki. J. Ikcr. C'. Brechtmann, J. Drc ule and S. Yt,unn~;s. J~'(~.6lh Cr'nf on lligh energy hca-ry-ion sludi¢~, a,fd 2nd Worksh~p on Anomalons, l..awrcnc¢ Ikrk¢Icy Lab<',rah:,ly,University ,.)f Calif,u,rnia (1983). []1] W . Heinrich, 1"[ I)rcchscl, W. ;:akowski. J. ['Jeer, ('. Brechlmann, J. Dreute and S. ~[a,.ag, Phys. Roy. Lelt. 52 0984) 14~1. [12] S. ~)nntaE, W. T;akowskl, W_ Hcinrich, J. l~cr and ]i.O 'I'itL¢l,| H[h int. Co~,nfic ray ('~m(., Hangalorc (:9~43) pap,:i c,xl~ '1 4-14.
02
FOR
In,,trtJmcnls
AN A .JTO~,IATIC M['ASURING
SYSTEM
P/~RTICLE
W . T ~ , A K O W S K 1 , B. S C H O F E R ,
J. D R I - U ' ] ' E . S S O N N ' I ' A G ,
aJltd Metl'~od,, i~J I)hy~,t ~. ~, Re>,ezr,.~h 2 2 5 (19~.~) 9L I(x) N~)tl|~- | I~!lland, A)lt~.lcrd:lnt
TRACKS
IN PLASTIC
C, B R E C H T M A N ' N .
DI£'IT.CI'ORS J. BE.E~,
1-I. D : E C I ~ S E I - a n d W . I ' I E I N R I C H IJ~i~er
T ~[ 3'tv.'Sen, Physics Department, P.O. Box 101240. D - 5 ~ O Si,.::~n, Fi, d. Rt,p. German)'
Receiv :1 6 I:'.'~ruary 1984
In [ ~e la~,! .e~" _'years p i c t t t r e a n a l y s i s meIho.d~, h a v e ¢v,.)lved iltl(., a I.)c,,,verfuI t , : c h ) l i q u e [ o r illl~a))tll'~'nl,-" 11'~ i~J t~uc.t-;,r Ir:tck:., iI~ |)[.i.,llt'
dclectt ;. Wv !~avc develop~l a micropr,~esso)-based picture an'lalysi.,,s~ .,,tt'm for .uuton:alic track mcasu.'cmenls. 'I [..-"vide() psctures of partiel, tracl . seer, through a microscope are digitized in real time an.I the picture a~nalysis i'~ done by software:. The mic)os.c()p~ is equipp.*l wil~, a sta~e drive~a by stepping motors, which are controlle( by a ~ parate unit:roproce.~or. A PI)P ! ' /23 supervises the operate ~n or :)ql microproce,;:.ors and stores the mea,,~urc.dd a l a o n it~ ma ,.) stora~,e devices. "l'hi,, paper de,,cribes II': ~ard- and ~.)ftv,arc of the .?-ste~) and the metla(_,ds of pit:lure analy_,,is which are u~a~d for t')e track identification and mea~uremen~l
1. |nlu-)duel i.)n Stt.,agl3 ionizing particles f o r m latent tracks of damal:: d t)):a~crial in nuclear track d e t e c t o r s like plastic foils c' gla,,,es. By etching of the detectors the surface is remo',,:':l v.~th a c o n s t a n t el;:hing velocity o,. li the dauna[ ~ ex,.eeds a threshold the m a t e n a l is etched a w a y a l o n g ~he r a i e c t o ~ " of .'.Ire pa,'ticle wtth a track etch vel{x::i'L, o, - ~:~.T h u s an ,;tch c,--ae is formed, which c a n be oh:, rve,! a n d m,'asttrt:d v,i!h a microscope. F o r c o n stant ;:chi~)g c o n d i t i o n s the length of this etch c o n e intreat ..:s '~,;tl" increasin~ radi:,tion d a m a g e , i.e. increasing ~.n .:rgy loss of the particle [1]. In : ty,)ical experiment u~i_ag plastic nuclear track detect, J.-; .::, stack of thin p~ ~.,tic foils (each of s o m e hund).':l /.:m thickness) i, eq)o,.:ed to a n a c c e l e r a t o r b e a m -:.f h:'avy ions or to '.!,c c.~smic radiation. C h a r g e s a n d n ~'ss~:- of heavy nuclei car' be d e t e r m i n e d f r o m the evzcrg.? Ins: as a function of their range. F o r this p u r p o ~ the le:,.,Fh.~ of the etch cones have to be m e a s u r e d . This was t:.:,ne ~n the past ~y h a n d . m e a s u r e m e n t s of the thre,6-,':,m,:rtsional co,ordinate,*, uf the etch cones. T h , use of p i c t u r e :maly:;i', systems for a u t o m a t i c m e a s c c m , : n t s of heav2¢ ion t<'acks in plastic n u c l e a r track tetcctors w a s rec~n~.l:.,' ,'Jescribed b y different ~uthol [;.-8]. T r a c k ntcasurc:ntents with video .'yslems using .,.ethods of picture an.'.)ly~L!sc a n be p e r f o u m c d if the n'.asLJrement is restricted t:~ a focal plane o n the surfac:, o1" the detcctor. F o r ~ ~,- ;topic by direct illumination t ; e l d p s e s which are fo aed b y the i n t e r s e t l i o n of the et .'.a c,)nes ',,.ith tl~e surf; :~: of the plastic material, appez, as d a r k areas o n a I::i~.hter b a c k g r o u n d . 'l'heir positi,:3s, areas, perimeters at:d o t h e r geometric qua,aLlties c.:n !re m e a s u r e d using "icture analysis systems_ 0167-. ')8"i/84/503.f~0 C Elsevi:.r Science Publishers B.V. (Nort -lt,-,iland Physics Publi:;i~ing D~vision)
l r o m the a r e a of the ellip~'s on the d e t e c t o r s u r f a c e the e n e r g y loss c a n be c a l c u l a t r d . [tov,~ever, these a u t o m a t i c ) a e a s u r e m e n t s are less precise if the etch figures *~e f~awed by a s u r f a c e s t r u c t u r e on the plastic foil_ O n the c her h a n d the use of c o m p u t e r i z e d i m a g e analysis t: ables the a p p l i c a t i o n of m~clear , r a c k d e t e c t o r s in (: p , ' r i r r e n t s collecting large u a n t i l i c s of d a t a , which '. :re in-possible until now. S t a n d a r d i m a g e analysis system,~, w h i c h are widely .ed in bio!ogy, medicine, r e m o t e sensing a n d material icnce, are c o m m e r c i a l l y available h)r c o m p u t e r i z e d , lea.,,uo~mcnts of n u c l e a r tracks, but they are rather ,:t),..nsive a n d unf'~exible. W c built :t p o w e r f u l systcna ' : sed on a real time video digitizer a n d a 16-bit micro;ocessor. T h e p i c t u r e analysis is d o n e b y c o m p u t e r . r o g r a m s that c a n be a d a p t e d to various ~pecial p r o b ~IIlS.
"l'l'~ s3,stent for a u t o ' 3 a t i c t r e k m~z).stluron~nts T h e a u t o m a t i c m e a s u r i n g device is s h o w n scl-.ematit a l l y in fig. 1. 'The plastic d e t e c t o r s are mounte.'l o n a micro_w,..ope stage, which c a n be m o v e d over 10 c m :,< 10 c m b y s t e p p i n g m o t o r s with an i n c r e m e n t a l step size of 0.25 /~m. A n a d d i t i o n a l s t e p p i n g m o t o r moves ~he fine a d j u s t m e n t of the micr~_)~ope fo(.'us. A c o n t r o l device allows ntotion of the stage u n d e r m a n u a l or c o m p u t e r control. 'I he particle track.-; seen t h r o u g h the m i c r o ~ o p e b y a video c a m e r a are identified a n d m e a s u r e d b y a picture analysis system. F o r this p u r p o s e the p i c t u r e is digiLizcd b y a video a d a p t o r a n d the d a t a are t r a n s f e r r e d to o n e of the p i c t u r e analysis c o m p u t e r s . "[he picture analysis
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a n d mea:'ured tracks c~n the foil frolla the cf>ortl61atcs of the respective field of view. T h u s the activities for the anlonla|~c track m e a s u r e m e n t are s h a l e d between different c o m p u t e r s which work parallel on differcnl tasks. T h i s m a k e s the system fast.
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T h e m o v e m e n t of the m i c r o s c o p e stage i~ controlled b y an B-bit m i c r o p r o c e s s e r M C 6[~02. Fig. 2 shows how this m i c r o c o m p u t e r is interfaced to different ,-'ompu.,ler~ts of the systenL A m o n i t o r a n d the software for ~,ta£'.e m o v e m e n t a n d c o n t r o l are stored in 8 khyte R O M . 2 kbyte R A M are available for d y n a m i c storage of flat;, a n d for special p r o g r a m s . "riffs c o m p u t e r is lioked to th,.: PEal:' via two serial intt:rfaces M C 6850 (ACIA}. These are tLsed for transfer of c o m m a n d s a n d d a t a or for d o w n l o a d i n g p r o g r a m s in m a c h i n e code w h i c h were gener,:ted in the P D P using a cross as,;embier. Six p e r i p h e r a l interface a d a p t o r s M(" 6~r20 (['IA) ar,.: used to COltnect the m i c r o p r o c e s s o r with tlh~ eb.'ctronic devices for stage m o v e m e n t . The x-)" Slelq.-'ing v.',,Hors are c o n t r o l l e d b y a unit which is available v:ith the m i c r o s c o p e stage. It c a n be used u n d e r c o m p u t e r control w h e n the pulses for the s t e p p i n g m o t o r s are. get,cra t e d b y software r u n n i n g on the M C 6802. By this the velocity of the stage m o v e m e n t cart easily t,e controlled a n d it is possible to define an acceleratio:l a n d a ciecele r a t i o n p h a s e for m o t i o n from point to point. "l his allows to move the stage with hJgh velocity "wizhoul losing steps. A s e p a r a t e c o n t r o l logic was inst,.lied ft~t the z-coordinate. In a d d i t i o n to the s t e p p i r ~ m o , o l s x~.e
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is d o n e by software. We u~,e four of these picture analysis c o m p u t e r s whicit c a n wock parallel o n different pictures T h e m a i n p r c ~ e s s o r o f the system is a P D P 1 1 / 2 3 It sV'rts o n e of the f o u r p i c t u r e a n ; l y s i s c o m p u t e r s w h i c h stores the picture in its m e m o r y . While the picture analysis c o m p u t e r is m e a s u r i n g the particle tracks, the P D P 1 ] / 2 3 t r a n s m i t s c o o r d i n a t e s of d i s p l a c e m e n t to the stage control, w h i c h then moves the stage to the next Eeld of view. A f t e r this the P D P s¢:lccts -', picture analysis c o m p u t e r w h i c h is idle a n d initializes it to take the next picture a n d to w o r k on it. Parallel to these activities the P D P has to collect the t r a c k d a t a detenrtined b y the different p i c t u r e analysis c o m p u t e r s a n d to calculate the a b s o l u t e co¢~rdinates of the detected
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; n s t a l l e l i w ' r e m e n t a l encoders. T h e i r signals arc fed mtc' co ruler.; that c a n be read by the nticroproccssor. This t o n b i n a t i o n of e n c o d e r s a n d s t e p p i n g m o t o r s provides a kigh reliability of stage m o v e m e n t . 1"he ~,:tage m o v e m e n t c a n al~) be p e r f o r m e d u n d e r nlanua! control. F o r tl'is p u r p o s e t h : x - y s t a g e c o n t r o l unit generates the pul~;es for the s t e p p i n g motors. "l-he frequency ol these pulses is defined by a joystick ~,'hich is installed o n the m a n u a l c o n t r o l b o a r d s h o w n schematically in fig. 3. The z - c o o r d i n a t e stepping, m o t o r c a n be m o v e d b y pressing o n e of the tv,-o b u t t o n s for z-movement. T h e m a n u a l control f a n c t i o n of the stage m o v e m e n t is n e c e s s a r y for setting up the system a n d for definition of a c o o r d i n a t e systern on the d e t e c t o r foil,;.
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Since we use f o u r picture analysis c o m p u t e r s in con, ninati0n with o n e video system, we need the video a d a p t o r shown in fig. 4 to link the TV system to the picture anal;~.,is c o m p u t e r s . In oL'r video system we us~a H a m a m a t s u C 10130 TV career ~ with a Saticon T u b e . which h.',s only a small lag of 2%. P a r t of this s y t e m is a c a m e r a control unit, which allo ,'s the c o r r e c t i o n of a s h a d i n g of £ae video picture. A ditionally it generates addresses to store the digitized picture into the c o m puter mentory. The.~e 18-bit a( Ir,".'~cs a n d Ihe video signal are fed into the video ada: tot, A T R W ;.0(J7 A D C digitizes -~ e video p i c t u r e in real nine within 40 ms. F o r 512 × 51. pic ~;e points (pixels) the grey lev,.:ls are d e t e r m i n e d v th :: reso,ution of 256 steps a n d sq,red in o n e byte. 'I le i , t f o r m a t i o n of two bytes is colh:cted in a late} an I th.~.,t transferred via D M A to the p c ' u r e an.'dys:s c~ mputer. Based on the 18-bit addre.,ses s u p p o r t e d b y t e c a m e r a control unit 24-bit addre, ses for the 256 kb.~ :e picture m c m o r y are generated v4thin the video ad plot. T o display the c o n t e n t s of ~he picture mcn-ory .:,n a video m o n i t o r "1 D A C is used. T h e video picture .; synchroni:~ed b y the address pulses generated b y the m c r a control u n h . The data bus a n d the 24-bit a ..:lre.ss bus of the video a d a p t o r can he switched be~we¢ I n s t p i c t u r e ;malysb, c o m p u t e r s . A switch-over i'.; a~ v a t : d by the p i c t u r e
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analysis processor.,, thenl~,elue~. A dur "rely I',:a,:l fl'lflll the highest a~dress activates th--' A D C • nd D M A logic a n d the d a t a of a digitized p i c t u r e are t r a n s f e r r e d to the p i c t u r e m e m o r y of the processor. T o start the d i s p l a y m o d e of the s y s t e m it is necessary to write a c o n s t a n t N to the highest a d d r e s s of the processor. This c o n s t a n t is stored in a register of the video a d a p t o r a n d initializes it to d i s p l a y the p i c t u r e m e m o r y for a time of N × 40 ms. W h e n the video a d a p t o r is linked to one of the f o u r p i c t u r e pr.x:es,~ors all rcquc,;ts of the other three pro~essors are ignored. 2.3. 27le picture analy.~is ( o m p u t c r v P'ip.. 5 ,,,Imws ,,thematically the: p i c t u r e anMysis c(mtpurer. T h e C P U is a ],"}-bit n t i c r o p r o c .sor M C 6~ly')O. Jt is s u p p o r t e d with 256 k b y t e p i c t u r e m e m o r y a n d the s a m e a m o u n t of p r o g r a m a n d d a t a m e m o r y . A d d i t i o n ally 24 k b y t e of R O M a n d 4 k b y t e of R A M are available for a residem m o n i t o r a n d :;tack o p e r a t i o n s . T h e a d d r e s s b u s a n d the d a t a bus of this c o m p u t e r can direclly be c o n n e c t e d to the video a d a p t o r as tleserihe(t above, T h e p r o g r a m c o d e for picture .',nalysis is generated by i,, cross a s s e m b l e r in the P D P . It c a n be d o w n l o a d e d into the p i c t u r e analysis c o m p u t e r vm a serial link. D u r i n g the p i c t u r e analysis all d a t a e x c h a n g e d between the M C 68000 a n d the P D P are t r a n s f e r r e d o n a parallel line. T h e pictnre analysis c o m p u t e r c a n be c~,nnected wi'_h the m a n u a l c o n t r o l b o a r d (fig. t). "lhi,, all.wn Io display a c u ] s o r in the picture rnt:nlc,ry which c a n be m o v e d by b u t t o n s o n the m a n u a l c o n t l o l b o a r d . T h e c u r s o r is g e n e r a t e d b y a special ~:,ftware w h i c h is initialized b y a c o m m a n d f r o m the P D P . A f t e r a take d a t a c o m m a n d f r o m the curs.or b o a r d the grey level o r the p i c t u r e p o i n t m a r k e d by the c u r s o r are d e t e r m i n e d a n d transferred to the P D P . This possibility is used w h e n
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with y ' the line n u m b e r of segment ~. T h e perimeter of :t track can be m e a s u r e d I.l:¢ sumt h i n g u p all peripheral points in units of pictur~ points. W e define the perimeter of a detected object as follov,'.-.: II
P = 2 n + l i + l , , + ~.[I,-I,_tl. p a r a m e t e r s for the p i c t u r e analysis p r o g r a m s have to bc set up for a nev.' type of d e t e c t o r foil.
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3.2. Sepuration of background 3.1. Stctn,'lurdprocedure for circular track; T h e particle track s h o w n in rig. 6a will b c used as an e.;anlple to explain tile s o f t w a r e for Irltek m e a s u r e m e n t . F o r this p u r p o s e the track win- analyzed o n l y with low n l a g n i f i c a l i o n so tllal the p i c t u r e analysis system sees an t)bjc.ct with only a "c~ picture points. Fig. 6b sho',~.s the section of d a t a c o r r e s p o n d i n g to this track, which are stored in the m e m o r y of the picture analysis system a f t e r digitizing_ T h e n u m b e r s give the grey levels of the p i c t u r e points_ Z e r o is black, 255 is white. If we define in this case a thresh~,ld value of 40 in grey level for particle tracks, the a r e a within the b o u n d : ries of the solid line s h o w n in fig. 6b is a c c e p t e d as a t:ack. T h e p r o c e d u r e of our p a t t e r n r e c o g n i t i o n p r o g r a m for detection a n d analysis of tracks is to e x a m i n e in a first step the stored video picture line by li",e s e a r c h i n g for s e g m e n t s that c o m p l y with the grey leve~ c o n d i t i o n . In o u r c l a m p % in fig. 6b 13 ,~egntertts are detected. The next step is to c o n s t r u c t objects f r o m the detected .,,cgments. T w o s e g m e n t s in a d j a c e n t lines b e l o n g to tile ',:tu~e object if they o v e r l a p in their x - c o o r d i n a t e s . F o r s e g m e n t s that d o n o t b e l o n g to a n object that was a l r e a d y detected in lines a b o v e a nev,' object has to be defined. If one object that w a s a l r e a d y detected earlier has no c o r r e s p o n d i n g ~ g m e n t in the actual line this object is defined as detected completely. F o r the detected objects some g e o m e t r i c quantities are calculated.
A m o r e c o m p l i c a t e d situation arit~es for the two small objects s h o w n in fig. 6a. We see on the detector s u r f a c e objects which are not d a r k in the centre. Cbjects of this type in m a n y cases are surfact, structure~. But also particle tracks s h o w a reflection of lig.h: qll the central p a r t if the), originate from s t o p p i n g p:lrlif:h~s or f r o m par'Jcles incident with small angles to the ,!erector surface. T h e picture analysis p r o g r a m s h a n d , " these objects, w h e r e the central grey levels are not h,.!,.~'.,.' IIte t h r e s h o l d of detection, like completely black objects. T h i s is achieved by the p r o c e d u r e , that the white g a p between two segm" Its in a line belonging to I|ie same object is fillt:d. S:ace v,,e lose some informaticr', a b o u t the measured object in this v,'ay, we additionally calculate the s u m of grey level values for a central N .'--:V pixel matrix, where N can be freely a d a p t e d . In combin a t i o n with the size of the object we have a cri.'.erion to distinguish particle tracks from detector surf~',:e strutlure.
3.3. Overlapping tracks P r o b l e m s arise from overla~_ning tracks like those s h o w n in fig. 7_ If the overlap is not ttx'~ large, they c a n be identified based o n the ratio of area a n d perimeter. W - defined some ~ o r e geometric quantities to identify these n o n circular objects_ The lengths of the projections d , , d,. of a detected
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[hC C¢IoltJI11¢1[¢2 ,IKL'Y,. lilt.' cov.,rl,mcC Cquals /elO. ]hl.lt [hL'['l ;l~.;llll llte d f f [ c r c n c c oI" the ~l,.)jcc[[Ol]% ~ a hl~.I]ll-ic~lil[ I[]dlL';II.IL,II For a r l o n - c l r c u l a r OhjCL'l l ~ o o v e r l a p p i n g c i r c u l a r track'. '-'an he .,ep.tr,llcd based o n an o p e r a t l o n called crohloil l-i~i, H'Ie,IIP, ,l ~ ] l n n k m g o f the COIIIOLIr o f an ~bject OlllllLIlt~ ~l)l lIL'n p h c r a l points. By ~,LIi.'Ce~Mve cr'.~sion o f Lwt, ~werlapp i n g tracks t w o s m a l l e r separated objects arc Im~duccd ] heir u'enlru o f ma.s~ t_-oordmat.cs arc Ihc c o o r d i n a t e s o f the [['ack~ Kl3~/~ll]g tlte degree o f clt)~,io[) thcd ,lrc~it~ O[ the e r o d e d objecl,s can hc corrected to get Lhe o r i g l n . d Iritck areas, l}a~ed on the ',echmque o f Seglll,~l}[:ilio,'1 ,oI d~c objects i n t o line :,cgmcn:s. w l u c h ~,: u:,e. ~' i~ po'.~iblc I~) d o l]t¢ ero.~lOn w l t l i ~larldard ¢}perator~ ;ix defmL'd h'~ ~(t)mlg el al. I9]. 3 4
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"]-ho particle Irilck~ %ho~.Lll ill ~1~ () LIIId 7 ~trc p l o duLed b?,. particla.,, perlt?lr,d]i~ the d e t e c t o r I ; e r p e n d ~ : w tar to its ~,url';,~:¢. T h e r e f o r e dle IIH,2r',ccHOI~ O[ I]Ic :2[~}i COliC i~.llh the d e t e c t o r ~tlrI'iLce Is a CIRIC ]'OT rlofl-XCrllcttl Irajc:c[orlcs thl% etch fJo,ure 1~ ,m clhp~c I,'.~' tile oiIc s h o w n llt fig. X. For thc'~c Irack~ the length ,::vJ ,~tv:atatlon of m i n o r a n d m a j o r axc~ p r o ; t d c tnlt.~rll~dl:O!l a b o u t the d i r e c t i o n of ~.he tra3ector', and the encrgx, !o~. o f the p a r n c l e W e c a n a n a l y z e elhpt]cal object:, b'. d e t e r n u m n g a d d m o n a l l y to the c o v a r i a n o . ' also the v.'natacc., l'lg 7 ".ltcropnotograph of t~o overlapping track~. I J-I
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F o r s o m e o p e l ' a l t o n : of the p i c t u r e analysis p r o g r a m s it is n , x e s s a r y to use floating p o i n t Operations. F o r this p u r p o ~ we d e v e l o p e d a floating p o i n t s o f t w a r e p a c k a g e for the M C 6 g O ~ wigh a p r e s e n t a t i o n of real a n d d o u b l e precision integer n u m b e r s b e i n g identical to the P D P 11 s t a n d a r d . T h a t m e a n s that all results of the p i c t u r e analysis c a n be directly used b y the P D P w i t h o u t f u r t h e r c o n v e r s i o n s . All floating fmint o p e r a t i o n s un the M ( ' 68000 arc a d i v a l e d hy line It)Ill e m u l a t o r Irap',. F o r 32-bit f l o a t i n g [mint variables in h i d d e n bit m.)rmalization a d d i t i o n a l l y to the f o u r basic :'rithmetic ~)pcrations the f o l l o w i n g o p e r a t i o n s are a,,;,.lable: c o m p a r e , negate, integer to floating p o i n t co,l,'ersion a n d vice versa. F o r 32-bit integer variables n m h i p l i c a t i o n a n d divisinn are defined. A d d i t i o n a l l y the s q u a r e r(×~l ftmctions for 32-bit integer a n d floating ~ i n t o p c r a u d s are available.
4.1. System sc4iware for the M C 6~ 'g'K) 4. 3. PActure anal|'si3 software F o r the pic,ure analysis comF_,ter we developed a special m o n i t o r c o n t a i n i n g system routines w h i c h simplif_y the o p e r a t i o n of this system u n d e r the c o n t r o l of the P D P 1 1 / 2 3 . T h e m o n i t o r c o n t a i n s some b a s c f u n c t i o n s like stalling a n d tracing p r o g r a m s rur, nin~ in user m o d e o r in supervisor mode, m e m o r y change: for byte, w o r d a n d Iongv,'t)rd and r u n n i n g a m,-.'mot3' t;::d p r o g r a m . A d o w n load function ( a n be used to ; , a n s f e r p r o g r a m s ira m a c h i n e (:ode ~enerut-.d b y a c r e l s a s s e m b l e r on the P D P into the m ; c r o p r o c c s s o r mern:}rv. F o r this p u r p o s e the M C 61~0<~'Jactivates a transfer ? r o g r a m on the P D P . Files cot:taining s-records c a n thug b e l o a d e d f r o m a n y ntass storage device of the P D P . ] ' h e addresses in the miclopro+.cssor m~.mory c a n be the ~ m e as defined in the .',-records or shifted by a c o n s t a n t , or c.m be c o n t i g u o as f r o m an a m b i g u o u s starting a.(dress. T o facilitate alter-native wor',: o n the P O P a n d the M C 68000 a m o n i t o r tune;ion ft-.r t r a n s p a r e n t terminal m o d e is available on the m i c r o p r o c e s s o r . T h i s allows the user of the m i c r o p r o c e s s o r terminal to w o r k w~th the P D P like on a terminal directly r a k e d to this m a c h i n e . A further g r o u p of m o n i t o r ft.=:tions enables tests of the system as a picture analysis ~,,'stem. These are basic functions for digitizing a picture or d i s p l a y i n g "he content of the picture nremor2," ~n .= T V m o n i t o r . F o r test purlx)ses dif[erem p a t t e r n s cat~ be s y m h e t l z e d in the memory. A f t e r a p r o c e s s o r reset the -nonitor is w a i t i n g for i n p u t from the P D P c,r f r o m the- terminal, d e f i n i n g the s l a t t i n g address of a program_ ']'he p r o c e s s o r reset c a n
T h e p i c t u r e a n a l ) s i s s o f t w a r e r u n n i n g on the M C 68000 follows a n idea developed b y H c n k e [2]. It consists of two b a d e p r o g r a m s , ] h e r o u t i n e " C O L U M N " d e t e r m i n e s detected line s e g m e n t s h a v i n g grey leveh. below or a b o v e a t h r e s h o l d a n d stores these d a t a into m e m o r y . T h e routine " F E A T U R E " r e c o n s t r u c t s detected objects b a s e d o n the d a t a p r o v i d e d b y " C O L U M N " . " I ' E A T U R E " calculates all the track d a t a de.scribed above. The ':ariable!; d e t e r m i n e d hy " C O I . U M N " a n d " F E A T I Y R E " are stored in a d y n a m i c r : x o r d s t r u c t u r e . T h e o v e r w h e l m i n g p a r t of the 256 k b y t e p r o g r a m m e m o r y is used for this p u r p o s e . T o analyze a p i c t u r e the P D P activates " C O I . U M N " to r u n or, rne M C 68000_ After c o m p l e t i o n of this p r o g r a m the P D P roceives s o m e s t a t u s a n d error c o d e s f r o m the M C 6tIIKIO a n d then starts the p r o g r a m "FI+]A TURi:."_ " F E A T U R E " also transrruts status c o d e s a n d the results of the p i c t u r e analysis. T h r e e different versions of " ' C O L U M N " are available. T h e fastest version is r u n n i n g with a c o n s t a n t dete~tion t h r e s h o l d of grey level. A s h a d i n g of the p i c t u r e w h i c h is not completely c o r r e c t e d by the video c a m e r a c o n t r o l unit c a n be c o n s i d e r e d hy versions of " C O L U M N " with n o n - c o n s t a n t det,mution tllreshulds. F o r a s h a d i n g that is c o n s t a n t for all sucoessive pictures the thresholds are defined b y a s h a d i n g matrix, H e r e the t h r e s h o l d s are given b y c o n s t a n t s for fields of g x 8 picture points. In the case that the s h a d i n g change~. f r o m o n e p i c t u r e to the next o n e a d y n a m i c threshold c a n be used. In this case a n individual threshold is
II/ "lrako, ~1~j el tJl. / A ut,)~nanc rpJeasurtptl~ r)'stem fi~r particle tracks c:~lculatcd f . r each sitlet~ lira: in the following way. The I:,, ,11 rlt:lxima of grey levels in the video line are linked togcthcJ J,v a p o l y g o n line. "[heft this is shifted b y a et:nstal-,t ~AI.se, I,~ f o r m the threshold for d e t e c t i o n of d a r k objects. T h e execution of the picture analysis by the p r o g r a m version with c o n s t a n t thrc:;hold i'~ p e r f o r m e d in 0 . 7 - 0 . 9 s. d e p e n d i n g o n the contelit of tht.: Fisture. The version with the s h a d i n g m a t r i x is a little ~kJwcr. it lakes a b o u t 1 2 s, w h e r e a s the p i c l u r e analysis using a d y n a n l i c threshcfld takes a b o u t 5 s p e r picture. All p i c t u r e a n a l y s i s p r o g r a m s were written in the as,~embler langu:~gc of the M C 6 8 0 ~ ) attd , r a n s l a t c d to alat,lline code USiltg a cross a s s e m b l e r r u n n i n g on tile PI31'. ¢.4. S o f t w a r e r u , m m g
on the P D P
T h e p r o g r a m s rnnrting o n the P D P use the hasic f u n c t i o n s of the p i c t u r e analysis s_vstcli~s a n d the stage corArol p r o c e s s o r for two m a i n p u r p o s e s . 1 testing the a u t o m a t i c m e a s u r i n g device a n d setting up p a r a m e t e r s for a special a p p l i c a t i o n , 2. p e r f o r m i n g a m e a s u r e m e n t for tracks on a d e t e c t o r foil. "I'o define the p a r a m e t e r s for a m e a s u r e m e n t d~ffere a t tests c a n be p e r f o r m e d . A c u r s o r c a n be m o v e d on the video screen a n d the grey levels at these positions c a n be read by the PDP. A d d i t i o n a l l y the grey levels a l o n g a video line c a n be d i s p l a y e d o n the screen as a d i a g r a m . Based o n this i n f o r l n a t i o n o n e of the three •,ersions of the p r o g r a m " C O L U M N " c a n be selected. To set u p the s h a d i n g matrix, a p i c ; u r e which is def¢c,:ttsed or is c o m p l e t e l y grey, with no i n f o r m a t i o n in it, c a n be digitized. r i t e ~hading m a t r i x d e t e r m i n e d in this w a y can be c h a n g e d b y a c o n s t a n t or b y the i n f o r m a t i o n of a n o t h e r picture w h i c h c o n t r i b u t e s with a ,,,,'eight f a c t o r betv,'een 0 a n d 1_ W h e n the detectior, threshold i,. defined, single piclures c a n be a n a l y z e J a n d detected objects c a n be m a r k e d by a w h i t e (or black) p e r i m e t e r a n d a central cross. T h e results of the track m e a s u r e m e n t s are p r i n t e d o n the ter,rfinal. T h e o p e r a t o r cart select tracks for w h i c h he believes the detectton to be problen'tatic a n d test the o p e r a t i o n o f the system fox these cases. A d d i t i o n a l activities to set up the system for meas u r e m e n t are the definition of the a r e a w h i c h hits to be s c a n n e d o n a plastic foil attd to m e a s u r e the m a g n i f i c a tion, that Ineans the size of one picture point in squ:tre |nicrtnls,. T h e measurennent of all tracks on a plastic d e t e c t o r foil is p e r f o r m e d b y m o v i n g flu m i c r o s c o p e s t a g e in m e a n d e r i n g path_ D u r i n g Ihis C r~eration the field of ,'Jew is sEiftcJ b y the size of the I old of view m i n u s an offset_ This offset is at least as lar ~- as the radins of the largest object which has to be m e a s u r e d . D u r i n g the
99
Iileastlrt.:nlent only objects with cc-ardinates inside a f r a m e of Llte size of this offset arc accepted. By this p r o c e d u r e we avoid to take d a t a oi tracks that are not c o m p l e t e l y inside the field of view. O n e m a j o r p r o b l e m is to contrail the focus of the m i c r o s c o p e , when a d e t e c t o r of larger size is SCalmed. F o r lc,wer magnifieatiol'l where this p r o b l e m is not as critical we d o this by a p p r o x i m a t i n g tl,c surface of the d e t e c t u r as :t plane. F o r this p u r p o s c the z-coordinates of three different points on the d e t e c t o r surface have to be m e a s u r e d d e f i n i n g the plane. D u r i n g the s c a n n i n g p r o c e d u r e the P U P calculates the a p p r o p r i a t e z-cc~_wdi~ate f r o m this plane. Thl,,, fast p r o c e d u r e c a n n o t be used for higher magr,,fication:, ,.,,here the focus has to be very pr-~cise. Here o l h e r method,, i~'.','e to he developed to define a focus f r o m the c o n t e n t of the picture. The sizes of the objects a> a function ol- Iht2 z - c o o r d i n a t e s h o w an e?.tlenlLinl at the p o i n t of focus. A n o t h e r q u a n t i t y for defining a focus is t_~le contrast c f the whole picture. T h e g r a d i e n t of the grey le',el values of a picture as a f u n c t i o n of ti'.c :'-co~wdinate I:as a p e a k at the focus position. U n f o r t u n a t e l y ~!au.,c p:-oced u r e s are rather slow and thus would reduce tile :,pc~'d of the m e a s u r e m e n t . The d e v e l o p m e n t of ai) alnofoctts for our system is u n d e r progress.
5. C o n c l u s i o n Tile m i c r o p r o c e s s o r based a u t o m a t i c picture an.d~',is s,:stem presented in this p a p e r i.,, a powerful toot f,~r m e a s u r i n g nuclear tracks in plast, ic detectors. The m:~l~r a d v a n t a g e of the system is its flexibility Since the picture analysis is p e r f o r m e d by software, we are a H e v.,~ a d a p t tile analysis m e t h o d s to special problems. The s y s t e r : is a p p l i c a b l e for a n a l y z i n g nuclear track detectors which ",,,'ere exposed to high particle fll:xes of accelerators. O n the other h a n d it c a n bc used also in c o s m i c ray experiments. A u t o m a t t c scantling is :ha adv~ n t a g c when dealing with low particle fluxes of cosrnic radiation_ Elliptical tracks of particles incident from various diieclions c a n be ffle~snred. W e have used this system in an experintent where 2 × 107 etch copes were m e a s u r e d to investigate the iftteraction m e a n free p.ilh of relativistic heavy iort b e a n i f r a g m e n t s [10,11]. F u r t h e r m o r e a special version of the zystem will be used in a cosmic ray e x p e r i m e n t [121 where it is necessary to analyze a b o u t 600 m-' of plastic track detectors. W.H_ t h a n k s E.V. Benton, R.M. Cassou a n d R.P. .-lenke for valuable discussions. This work is s u p p o r t e d b y the B u n d e s m i n i s t e r fur F o r s c h u n g u n d Technologic, No. 01QV3127. No. 01QV2032, N o 01QV3029 a n d No. 01QV2130.
1(~
If~ Tro~[ow; ~.'i et .L / .( ut<,-!',t1( mee.rurinf4 .;ysrcm f~r piirll( h Ivtl('k s
Referer,~s [I.] R.L. Flcb,chcr, P.B. 1~c¢ ard R.M, 'Walk¢r. Nu.:Icar tracks in solids ,Univcrsily of Ca!i[ornia Pre~, 1975~, [2] R.P. Her, k¢, E.V. i].cnt(:q znd R.M. Cassou. Solid slate nuclear Irack d,~,ec|ors .'Pcx[,ar:on, New York, 1980) p. 509, and private discussiox ': [3] E.X. Shirk and P.B. Prier 16~h Int. Cosmic ray Conf. Kyoto, Japan, vol. 11. (19;9j -'~. ['6. [4] J.H. Adams. Nucl. Tracks 4 [1980) 67. [S] W Trakowski, B. ~.~ch6fer anP W Heinrich. CERN rc'Tw,rt 8,,-07 (1981) p_ 44~. [6] J. Berr. J. Pipper and W. H¢.nri:h, J. Phys. E 15 (1982) 439. [7l W_ Hei~rich. W. Trakowski, , )icr.r and It. ,'~-huchl.. Pr~.'. ] l t h Int. Conf. on ~)lid sl, I~. nue'.ear tr;,ck de!.s=tor,,, Bristol (]9M) p. 9 l l .
[k I W Heinr~,.h. II. D-et.h~l, It. Rudat, I" V. l:knton ~,ml [). ['|iILtebrand Nu,c'L nslr. mnJ M¢th. l'.~J(19141) .169. [gJ I.T. Youna. ft.[.. Peverim. P.V,. 'Vcrh,eck and P J. ",'on Olterlu,o, C o m p . (..;"aph. ln~ds¢ Pr(~. 17 (19Rl) 1:4~. {It)} W. lleinnch, I']. Dr,cchsci, W. Tr*kow:,ki. J. Ikcr. C'. Brechtmann, J. Drc ule and S. Yt,unn~;s. J~'(~.6lh Cr'nf on lligh energy hca-ry-ion sludi¢~, a,fd 2nd Worksh~p on Anomalons, l..awrcnc¢ Ikrk¢Icy Lab<',rah:,ly,University ,.)f Calif,u,rnia (1983). []1] W . Heinrich, 1"[ I)rcchscl, W. ;:akowski. J. ['Jeer, ('. Brechlmann, J. Dreute and S. ~[a,.ag, Phys. Roy. Lelt. 52 0984) 14~1. [12] S. ~)nntaE, W. T;akowskl, W_ Hcinrich, J. l~cr and ]i.O 'I'itL¢l,| H[h int. Co~,nfic ray ('~m(., Hangalorc (:9~43) pap,:i c,xl~ '1 4-14.