- Email: [email protected]
Results from a hybrid silicon pixel telescope tested in a heavy ion experiment at the CERN omega spectrometer
I | l [ I l 1;I I | I i'ii~l[151 q
PROCEEDINGS SUPPLEMENTS
RESULTS
FROM ION
Nuclear Physics B (Proc. Suppl.) 32 (1993) 260-268 North-Holland
A HYBRID
EXPERIMENT
CERN
SILICON
PIXEL
AT THE
CERN
TELESCOPE OMEGA
TESTED
IN A HEAVY
SPECTROMETER
D e t e c t o r R~'4D C o l h , b o r a t i o u R D - 1 9 , p r e s e n t e d by M . G . C a t a n e s i
M,G. C a t a n e s i 5), H. Beker s) , W. BeuschU, M. Carol)bell t), E. Chesi 1), J.C. Clemens 2), P. Delpierre 3), D. DiBari 5), E.H.M. HeijneU, P . . l a r r o n t), V. Lenti '5), V. Manzari 5), M. M o r a n d o 7), F. Navach 5), C. Neyer 4), F. Pengg 1), R. Perego ~;), M. Pindo (;) E. Quercigh 1), N. R.edaelli ¢I), D. Sauvage 2), -t
1-
t~
(,. ,q_'ega.to7), S. SllllOlle a)) Abstract A telescope made of three OMEGA-ION hybrid silicon pixel detectm's has been successfully tested in the heavy ion experiment WA94. Each plane consisted of a single detector with IOOG active l*ixels (500 /tm x 75 Itm), eadl one being bump-bonded to the read-out chip,and arranged in 16 cohntms and 63 rows respectively. With a sensitive area a.~ small ,~ 8000x4725/tin 2 several million event.~ with at least one track originating fi'om sulldmr-sulldmr interactions have been recorded in a few homw. Results on target reconstructiott, tracking acclzracy attd efliciem:y are presented.
1
CERN, CH-1211 Geneve 23, Switzerland
2 3 4 5
CPPM ,Marseille, France College de France, Paris, France Eidgenossische Technische H,,chschule Zurich, Switzerland INFN and University of Bari, Italy INFN and University .f Milano, Italy INFN and University of Pad.va, Italy INFN La Sapienza, [Aome, Italy
7 s
0920-5632/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.
CERN Detector R&D Collaboration RD-19 ~Hybrid silicon pixel telescope tested
261
1. Introduction
2. Experimental conditions
In particle physics, particulary in high multiplicity and high rate environments like heavy
During the run of the heavy ion experiment WA94, a I)arasitic test of the pixel telescope has been performed in the 1.8 Tesla magnetic field of the CERN OMEGA spectrometer. A sulphur I)emn hit a sulphur target, with an intensity of 5-105 partMes/spill and a spill length of 5.1 s every 19.2 s. The experimental set-up is sketched in fig. la: three pixel detector pla.nes were positioned downstream of the target at 204,224,235 mm distance, with their centres 11.5 mm below and 3 mm sideways of the beam axis. In such a way the largest pa.rt of the produced particles is not, detected and radiation damage did not, occur in
ion experiments and filture hadron colliders, the track detectors in the inner region need a precision of 10 # m [1]. The approach using silicon microstrip detectors may satisfy this precision, but in view of the computation time used for resolving the ambiguities should not be acceptable. A solution can be to complement the microstrips with 2-dimensional silicon pixel detectors: such devices present a finer segmentation and lower noise [2]. A first effort in the development of silicon pixel detectors with adequate speed was performed in the framework of the CERN-LAA research and develoI)ment progra.m [3] and now this work continues in the R.D19 program [4] . The new prototype a.rray of 16x63 active pixels with asynchronously strobed binary output presents a silicon a.rea of 8.3mm x 6.(i ram, significantly larger than in [a]. The first stage of the signal processing electronics, incorporated in an area equivalent to the detecting one, is connected to the detector using tile bump bonding technique [3] [4] [5]. The first stage consists of the amplifier and the comparator followed by an adjustable digital delay element, a strobed multiplexer and a D flil)-flop. A detailed description of this prototype is given elsewere [5] [6]. Three such device,~ each consisting of 1006 pixels arranged in 16 cohmms and 63 rows respectively, have been successfldly tested in tile heavy ion experiment WA94 at CERN [5]. Here we present the results from the offline analysis of the data recorded during this test.
this test. Part of the rmming time was devoted to the study o1"tile a.djustment of two hardwa.re parameters, tile threshold and the strobe delay [6]. Tile wdues chosen fbr tlle currents regulating the threshohl (ids2) and tile strobe delay (idn) are 30 8.S/tA respectively. The results presented in this paper come from 5.105 events which have been collected tamer stable, optimized conditions in 8 hours. Only 4 noisy pixels have been found out of 3018 and they have been removed from the analysis which is presented in this paper. No dead pixel has been detected.
irA and
262
target 3,,S J ,
CERN Detector R&D Collaboration RD-19 ~Hybrid silicon pixel telescope tested
...204ram
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*
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3 20
-
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8
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Fig. 1. a) Sketch ~f t h e e x p e r i m e n t a l s e t - u p , b) Pr~bjecti~m
4
8
on t h e x-z p l a n e ~*f t h e t r u n c a t e d pyr~mlid used t4J select t h e e v e n t s ill otlr a n a l y s i s . T h e d r a w i n g s are n o t t~* sca]e.
12
16
12
16
Plane 3 6O
3. Analysis 20
Since the three detectors are not arranged in a
!
projective way a fiducial volume h~s been defined in order to choose tracks which tra.vel through the whole telescope. We select a subset of our data by requiring that all hit pixels in one event be inside a voluine defined ~s a pyramid pointing to the target and containing the common region of the three detectors (fig. lb). Moreover we exclude events with hits in the regions corresponding to columns 1 to 4 m plane
planes representing the number of hits detected ill tile fiducial vohune is shown in fig.2.
3, because a mistuning of the electronic bi~s current called idn for this plane may have caused inefficiencies. At this stage of the analysis no constraints are imposed on the hits. About 22 % of the events survive this cut. The ma.ps of the three
fig.3 shows tile distribution of tile number of hit pixels per pla.ne and per event. Because of the very small solid angle covered by the telescope, which is placed off the I~ea.m line, the mean value of the distribution is only 1.7 hits. Only for few
4
8
Column number Fig. 2. D i a g r a m s r e p r e s e n t i n g t h e m l m b e r of hits d e t e c t e d for each pixel in t h e p l a n e s 1,2 a n d 3 after t h e c u t . T h e h.riz~,ntal l e n g t h ~f each |rex is p r ~ l m r t i o n a l to t h e n u m t,er of hits.
CERN Detector R&D Collaboration RD-19 ~Hybrid silicon pixel telescope tested
~
t-
-+ Clusterof width= 1
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263
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---} Clustero! width= 2
--2
10
-3
i
i
10
J
Y column
o)
-4
10
0
20
40
Number
60 80 100 of hit pixels/plane
Fig. 3. Distrilmtion of the numl,er of hit pixels per plane allfl per eveltt.
E
0
0.75
events the multiplicity is larger than 20 hit pixels per plane (the occupancy is always less than 5%). A simple topological and 1-dimensional alghorithm h ~ heel| used to study adjacent hit pixels along z (i.e. in the same column, see fig.4a). This simplification was possible because of the different granularity of the detector in the two dimensions (the pixel y-dimension is more than 6 times larger than the z-one). We define as "cluster" a group of hit pixels which are adjacent along z and we call "cluster width" the nmnber of hit pixels. For example, an isolated single hit pixel is a cluster of width one. fig.4b shows the cluster width distribution of our data: 79 % of the clusters has a single lilt, 19 % h~Ls a double hit, 1.9 % has a width between 3 and 10, and only 0.i % ha.s a width larger than 10. One should kee l) in mind that there is a possibility of high multiplicity ill the heavy ion reactions. The mean width of our clusters is about l.a pixels. Tile observed percentage of double hit events corresponds to ttle charge sharing expected for adjacent pixels when crossed by inclined tracks, and simi)ly taking into account the geometry of the detector (which is 300 i t m deep with a pixel t.)itch of 75 Inn)
~
0
<--- Single
Hit
0.5
0.25
0
1
2
3
4 5 6 7 b) Cluster width
Fig. 4. a) Schematic picture illustrating our cluster definition: in this e x a m p l e a cluster of w i d t h = l (single hit) and w i d t h = 2 {double hit)are s h . w n . I:,) Cluster width distributi.n.
4. (;eometrical Reconstruction The frost way to reconstruct the target position is to select candidates to be single track events for which no complicated reconstrtlction l)rogram is needed . The selection is simply done by choosing events with one cluster per plane. This kind of events represents about 1/3 of our salnple (around 1.2.1() 4 events). Since we do not have any external detector to check the relative position of our telescope planes, we have aligned these planes requiring the self consistency of the offsets. First we choose a ,:lean sample of events having only a single hit per plane, then we re-
264
CERN Detector R&D Collaboration RD-19 / Hybrid sificon pixel telescope tested
quire that the track candidate comes from the
Z
interaction region (target). Therefore, each plane is aligned by minimizing the X2 distribution of
r
the trajectory fit of all track candidates. After
x
4
-b/,~
alignment, a linear fit to the tracks has been performed in the zx plane (minimally affected by the
a)
Plane 1
2 3
magnetic field) using the baricenter values of the clusters (fig.5a). The distribution of the intercept between the fitted tracks and the axis z=0 is given in fig.Sb: the aceumula.tion in the target region proves that tracks are coming from physical
"~'120o u
interactions.The tail at negative x corresponds to upstream interactions not filtered by a simple volumetric cut (some m a t t e r is indeed present on the b e a m line in front of the target). This distribution has been fitted assmning a gaussian
d
H 800
x "13
g ¢-
400
"I3
shape for the target region, superinq)osed over 0
a flat backgromKl. IInder these assumi)tions the
-8
target, position can be recontructed with a cr of
-4
0
4
b)
8
12
15
-b,o (c~)
1.035 cm. It is now possible to evaluate the spatim accuracy of the telescope by using the same
Fig. 5. a) Sketch of the a p p a r a t u s in the xz plane; the
sample its for the target reconstruction. First we
of the linear fit ( - b / a ) r e p p r e s e n t the intercept along the
define a straight line with the baricenter vMues of the clusters in planes 1 and 3 . Then we compute
fiw 12745 events with one c l u s t e r / p l a n e : T h e function
the difference Dz between the baricenter value of the cluster in plane 2 and the intercept of the line
I'ati~* (sign inverted) between the c o n s t a n t and the slope b e a m (x) axis. b) Distril,ution ,ff the ' - b / a ' p a r a m e t e r F = 995.- e - ° ' 5 ' ( ( x - l ' 2 4 ) / l " ° a s ) ~ q- 116.5 is used to fit the distribution.
with this detector. The distribution of the residual Dz is presented in the fig.(i. This distribution was fitted assuming a, ga.ussian shape. The mean value o f - 3 3 t m l rei)resents our error on the align-
5. Efficiency
nlent of the middle l)lane. The cr of 25#m is a good estimate of our precision: it compares well with the one expected for an ideal telescope with 3 planes and 75ttm of pitch, which is cah:ulated to be 75/(2 * v ~ ) = 22tim.
The knowledge of the tracking accuracy together with tile variance of the linear fit can be use
We call variance tile s u m ~f the s q u a r e d residuals indicating the b a d n e s s c~f fit. If the variance is < 1 0 - 4 c m 2 all the residuals are < 1001tin. In o t h e r words we can say t h a t the 3 points are aligned within t,m" tracking precision. If the variance is > 10 -4 cm
CERN Detector R&D Collaboration RD-19 ~Hybrid silicon pirel telescope tested
e4
2
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//
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-1.2
with one c l u s t e r / p l a n e : A gaussian Mml)e is used to fit
the d i s t r i b u t i o n
In such a way we select 11808 events (around the 94 % of our sample) fig.7a, 7b, and 7c show the correlations between the z coordinates of the three planes combined two by two, while fig.Td shows the z-coordinates in the middle plane versus the point where the line constructed using the z coordinates in the plane 1 and 3 intersects the middle detector. The wMth of the distributions can be explained by the angular spread of the partMes emerging from different intera.ction points of the target. We have studied the remaining 6 % in order to understand the nature of the so called "bad" canditates. We have subdivided this sample in two subsamples according to the value of the variance of the linear fit. In fact it, seems that this difference reflects a different interpretation of the data. 151 fig.8 we show the same coordinate correlation plots for candidates with a very bad variance of the linear fit(> 10 -e cm) and a D z > 3or (300 events,around 2 %). The interpretation of the data is that these are 2 track events not fully contained in our fiducial volume. As sketched in fig.9, the first track interceI)ts planes 2 an(l 3 this m e a n s t h a t one point is n~Jt ~m a s t r a i g h t line
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Fig. 7. Z-coordinat, es c~wrelation plots tiw the s(~ called "g~md events": zi i : l , 3
is the baricenter value (cm) of
the cluster ~Jn the plane i, z2F is tim z intercept on the middle detect~,r with the line m a d e using zl and z3. a) z2 versus zl. b) z3 versus zl. c) z3 versus z2. d) z2F versus z2
(but not 1), the second only plane 1. Those candMates are false single track events and should be removed from our sample when calculating efficiency. The second subsample contains candidates with a variance oil the linear fit in tile range 10 -4 < v a r < 10 -~ and a D z > 3or (600 events,around 4%). Those events seem to be low energy particles (like protons around 1 GeV ) which i)resent a big scattering in the middle plane. In this case, in fact, the correlation plots (figl0) show a good correlation between planes 1 and 2 and between planes 2 and 3 but a missing correlation between planes 1 and 3 because of the large scattering angle. The fraction of events found is consistent with the analytical evaluation of the probability that a proton of 1 GeV energy has a scattering in the middle plane leading to a deviation of 100/tin or more.
266
CERN Detector R&D Collaboration RD-19 /Hybrid silicon pixel telescope tested
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Fig. 8. Z-coordinates correlation l)lots f(n" the events with vat" > 10 -2 cm and Dz > 3¢r zi i=1,3 is the baricenter value (cm) of the cluster on the l)lane i, z2F is the z intercept on the middle detector with the line made using zl and z3. a) z2 versus zl. b) z3 versus zl. c) z3 versus z2. d) z2F versus z2 Only a correlati,m between z2 and z3 in the detectors right-up ,'orner is visible.
-
1.25
(c)
-)1
z2
-
"' 1.25
'
'
(d)
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Fig. 10. Z-c,u)rdinates c,,rrelation ph,ts for the events with 1 0 - 4 < vat < 1 0 - 2 and Dz > 3a zi i--1,3 is the baricenter value (cm) of the cluster ,m the plane i, z2F is the z intercept ,m the middle ,letect,w with the line made using zl &lid Z3. a) Z2 vet'stlS zl. b) z3 V e l ' S t l S zl. ,:) z3 v e r s t l S z2. d) z2F versus z2 line defined I)y these two c o o r d i n a t e s i n t e r c e p t s the b e a m axis in t h e t a r g e t r e g i o n as p r e v i o u s l y defined. T h i s r e q u i r e m e n t is very i m p o r t a n t for t h e s a m p l e w i t h o u t h i t s in t h e m i d d l e p l a n e , ill order to strel-lgthen tile d e f i n i t i o n of a real "track".
Fig.lla
shows a d i s t r i b u t i o n of this
b e a m axis i n t e r c e p t (in t h e t a r g e t r e g i o n ) , to be c o l n p a r e d w i t h f i g . l l b , where a n e n l a r g e d view of the corresl)ortding d i s t r i b u t i o n t a k e n Fig. 9. Schematic view in the x-z plane ,)f one event with one cluster /I)lane generated I)y two tracks.
from
fig.5 is s h o w n . T h e two s h a p e s are very s i m i l a r ( even if t h e n u I n b e r of e n t r i e s is q u i t e different). If we call " N e m p t y " t h e n u m b e r of e v e n t s
T h e efficiency is t h e l ) r o b a b i l i t y t h a t a t r a c k t r a v e r s i n g a pixel d e t e c t o r
will
a c t u a l l y be de-
t e c t e d . A n e s t i m a t e of t h e efficiency for p l a n e 2 was m a d e by a p p l y i n g a s i m i l a r c r i t e r i o n as
in t h e s a m p l e w i t h n o hits in t h e m i d d l e p l a n e a n d " N f u l l " t h e m u n b e r of e v e n t s in t h e s a m p l e w i t h one c l u s t e r per p l a n e , t h e efficiency for the m i d d l e l)lane is defined a,s:
for " g o o d " c a n d i d a t e s , i.e. o n e c l u s t e r in p l a n e 1 a n d 3, a n d n o r e q u i r e m e n t for the m i d d l e p l a n e . M o r e o v e r , a c a n d i d a t e is a c c e p t e d if t h e s t r a i g h t
E f f i c i e m c y = N f ull+ N/,,u N e m p t y = 99.2%
CERN Detector R&D Collaboration RD-19 /Hybrid silicon pixel telescope tested
267
References [1] P.E.Karchin, Use of pixel detectors in eleinentary particle iA~ysics,Proc.2ml.Int. Workshop on silicon pixel detect.rs,Leuven 1990 ,Nucl.Instr, Meth. A305 (1991 )497. [2] P.Jarron, Fast silicon detector systems for high luminosity ha
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|
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1
2
3
[3] M.Camphell et al., A 10 MHz micropower CMOS tY, nt end for direct rea(hmt of pixel detectm's,Nucl.Instr, and Meth. A290(1990) 149. [4] W.Beusch et al., C E R N / D R D C 90-81,P22(RD19),
10 8
II
{3
"-"
4
r"O
2
~M eon2 1.2~2 ~~ 6 "[,..,
0
I
I
I
I
o
1
2
5
(b)
cm
Fig. 11. a) Distrilmtiml of the I,eam axis intel'cept f~,r events with one cluster/plane, b) Distribution . f the beam axis intercept f~r events with one cluster in the planes 1 and 3 and m~ hits in the middle plane, hi hoth cases a gaussian shape is used t~ fit the
RD Prolmsal flu" the devel~pment of hybrM and m.m,lithic silicon micropattern detectors, CERN,Cleneve,Sp.kesman E.H.M. Heijne [5] F. Anghimdfi et al., C E R N / E C P 91-26 ;A 1006 element hybrid silicon pixel detector with strobed binary output:presented by Erik.H.M. Heijne at the IEEE Nucl.Science Symposium 1991 5-9 November 1991,Santa Fe,USA to be published on IEEE.Trans.NucI.Sc Ns-39(1992) [[;] F. Anghin.lfi et al., CERN E C P / 9 2 - 6 Design and Performances . f the OMEGA-ION hybrid silicon pixel detector:presented by M.(?ampbell at the 6th EIIl'Ol)eall S y l l l p o S i l l l l | Oll S e n l i c < m d u c t o r
24-2~; Fehruary 1992,Milano,Italy
6. Conclusions For the first time a test of a hybrid silicon pixel detector wms carried out succesfully in a fixed target exI)eriment environment. It w~Ls possible to develoI.) a very simple and thst pattern recognition, without ambiguities ill space, for tile study of single and multitrack events. Tile test
has shown the good position accuracy of the pixel detector,whic, h was 25 ibm in the direction of the 75 # m pitch. The etficiency was measured to be 99.2%.
Detector
PROCEEDINGS SUPPLEMENTS
RESULTS
FROM ION
Nuclear Physics B (Proc. Suppl.) 32 (1993) 260-268 North-Holland
A HYBRID
EXPERIMENT
CERN
SILICON
PIXEL
AT THE
CERN
TELESCOPE OMEGA
TESTED
IN A HEAVY
SPECTROMETER
D e t e c t o r R~'4D C o l h , b o r a t i o u R D - 1 9 , p r e s e n t e d by M . G . C a t a n e s i
M,G. C a t a n e s i 5), H. Beker s) , W. BeuschU, M. Carol)bell t), E. Chesi 1), J.C. Clemens 2), P. Delpierre 3), D. DiBari 5), E.H.M. HeijneU, P . . l a r r o n t), V. Lenti '5), V. Manzari 5), M. M o r a n d o 7), F. Navach 5), C. Neyer 4), F. Pengg 1), R. Perego ~;), M. Pindo (;) E. Quercigh 1), N. R.edaelli ¢I), D. Sauvage 2), -t
1-
t~
(,. ,q_'ega.to7), S. SllllOlle a)) Abstract A telescope made of three OMEGA-ION hybrid silicon pixel detectm's has been successfully tested in the heavy ion experiment WA94. Each plane consisted of a single detector with IOOG active l*ixels (500 /tm x 75 Itm), eadl one being bump-bonded to the read-out chip,and arranged in 16 cohntms and 63 rows respectively. With a sensitive area a.~ small ,~ 8000x4725/tin 2 several million event.~ with at least one track originating fi'om sulldmr-sulldmr interactions have been recorded in a few homw. Results on target reconstructiott, tracking acclzracy attd efliciem:y are presented.
1
CERN, CH-1211 Geneve 23, Switzerland
2 3 4 5
CPPM ,Marseille, France College de France, Paris, France Eidgenossische Technische H,,chschule Zurich, Switzerland INFN and University of Bari, Italy INFN and University .f Milano, Italy INFN and University of Pad.va, Italy INFN La Sapienza, [Aome, Italy
7 s
0920-5632/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.
CERN Detector R&D Collaboration RD-19 ~Hybrid silicon pixel telescope tested
261
1. Introduction
2. Experimental conditions
In particle physics, particulary in high multiplicity and high rate environments like heavy
During the run of the heavy ion experiment WA94, a I)arasitic test of the pixel telescope has been performed in the 1.8 Tesla magnetic field of the CERN OMEGA spectrometer. A sulphur I)emn hit a sulphur target, with an intensity of 5-105 partMes/spill and a spill length of 5.1 s every 19.2 s. The experimental set-up is sketched in fig. la: three pixel detector pla.nes were positioned downstream of the target at 204,224,235 mm distance, with their centres 11.5 mm below and 3 mm sideways of the beam axis. In such a way the largest pa.rt of the produced particles is not, detected and radiation damage did not, occur in
ion experiments and filture hadron colliders, the track detectors in the inner region need a precision of 10 # m [1]. The approach using silicon microstrip detectors may satisfy this precision, but in view of the computation time used for resolving the ambiguities should not be acceptable. A solution can be to complement the microstrips with 2-dimensional silicon pixel detectors: such devices present a finer segmentation and lower noise [2]. A first effort in the development of silicon pixel detectors with adequate speed was performed in the framework of the CERN-LAA research and develoI)ment progra.m [3] and now this work continues in the R.D19 program [4] . The new prototype a.rray of 16x63 active pixels with asynchronously strobed binary output presents a silicon a.rea of 8.3mm x 6.(i ram, significantly larger than in [a]. The first stage of the signal processing electronics, incorporated in an area equivalent to the detecting one, is connected to the detector using tile bump bonding technique [3] [4] [5]. The first stage consists of the amplifier and the comparator followed by an adjustable digital delay element, a strobed multiplexer and a D flil)-flop. A detailed description of this prototype is given elsewere [5] [6]. Three such device,~ each consisting of 1006 pixels arranged in 16 cohmms and 63 rows respectively, have been successfldly tested in tile heavy ion experiment WA94 at CERN [5]. Here we present the results from the offline analysis of the data recorded during this test.
this test. Part of the rmming time was devoted to the study o1"tile a.djustment of two hardwa.re parameters, tile threshold and the strobe delay [6]. Tile wdues chosen fbr tlle currents regulating the threshohl (ids2) and tile strobe delay (idn) are 30 8.S/tA respectively. The results presented in this paper come from 5.105 events which have been collected tamer stable, optimized conditions in 8 hours. Only 4 noisy pixels have been found out of 3018 and they have been removed from the analysis which is presented in this paper. No dead pixel has been detected.
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3. Analysis 20
Since the three detectors are not arranged in a
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projective way a fiducial volume h~s been defined in order to choose tracks which tra.vel through the whole telescope. We select a subset of our data by requiring that all hit pixels in one event be inside a voluine defined ~s a pyramid pointing to the target and containing the common region of the three detectors (fig. lb). Moreover we exclude events with hits in the regions corresponding to columns 1 to 4 m plane
planes representing the number of hits detected ill tile fiducial vohune is shown in fig.2.
3, because a mistuning of the electronic bi~s current called idn for this plane may have caused inefficiencies. At this stage of the analysis no constraints are imposed on the hits. About 22 % of the events survive this cut. The ma.ps of the three
fig.3 shows tile distribution of tile number of hit pixels per pla.ne and per event. Because of the very small solid angle covered by the telescope, which is placed off the I~ea.m line, the mean value of the distribution is only 1.7 hits. Only for few
4
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Column number Fig. 2. D i a g r a m s r e p r e s e n t i n g t h e m l m b e r of hits d e t e c t e d for each pixel in t h e p l a n e s 1,2 a n d 3 after t h e c u t . T h e h.riz~,ntal l e n g t h ~f each |rex is p r ~ l m r t i o n a l to t h e n u m t,er of hits.
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events the multiplicity is larger than 20 hit pixels per plane (the occupancy is always less than 5%). A simple topological and 1-dimensional alghorithm h ~ heel| used to study adjacent hit pixels along z (i.e. in the same column, see fig.4a). This simplification was possible because of the different granularity of the detector in the two dimensions (the pixel y-dimension is more than 6 times larger than the z-one). We define as "cluster" a group of hit pixels which are adjacent along z and we call "cluster width" the nmnber of hit pixels. For example, an isolated single hit pixel is a cluster of width one. fig.4b shows the cluster width distribution of our data: 79 % of the clusters has a single lilt, 19 % h~Ls a double hit, 1.9 % has a width between 3 and 10, and only 0.i % ha.s a width larger than 10. One should kee l) in mind that there is a possibility of high multiplicity ill the heavy ion reactions. The mean width of our clusters is about l.a pixels. Tile observed percentage of double hit events corresponds to ttle charge sharing expected for adjacent pixels when crossed by inclined tracks, and simi)ly taking into account the geometry of the detector (which is 300 i t m deep with a pixel t.)itch of 75 Inn)
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4. (;eometrical Reconstruction The frost way to reconstruct the target position is to select candidates to be single track events for which no complicated reconstrtlction l)rogram is needed . The selection is simply done by choosing events with one cluster per plane. This kind of events represents about 1/3 of our salnple (around 1.2.1() 4 events). Since we do not have any external detector to check the relative position of our telescope planes, we have aligned these planes requiring the self consistency of the offsets. First we choose a ,:lean sample of events having only a single hit per plane, then we re-
264
CERN Detector R&D Collaboration RD-19 / Hybrid sificon pixel telescope tested
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Fig. 5. a) Sketch of the a p p a r a t u s in the xz plane; the
sample its for the target reconstruction. First we
of the linear fit ( - b / a ) r e p p r e s e n t the intercept along the
define a straight line with the baricenter vMues of the clusters in planes 1 and 3 . Then we compute
fiw 12745 events with one c l u s t e r / p l a n e : T h e function
the difference Dz between the baricenter value of the cluster in plane 2 and the intercept of the line
I'ati~* (sign inverted) between the c o n s t a n t and the slope b e a m (x) axis. b) Distril,ution ,ff the ' - b / a ' p a r a m e t e r F = 995.- e - ° ' 5 ' ( ( x - l ' 2 4 ) / l " ° a s ) ~ q- 116.5 is used to fit the distribution.
with this detector. The distribution of the residual Dz is presented in the fig.(i. This distribution was fitted assuming a, ga.ussian shape. The mean value o f - 3 3 t m l rei)resents our error on the align-
5. Efficiency
nlent of the middle l)lane. The cr of 25#m is a good estimate of our precision: it compares well with the one expected for an ideal telescope with 3 planes and 75ttm of pitch, which is cah:ulated to be 75/(2 * v ~ ) = 22tim.
The knowledge of the tracking accuracy together with tile variance of the linear fit can be use
We call variance tile s u m ~f the s q u a r e d residuals indicating the b a d n e s s c~f fit. If the variance is < 1 0 - 4 c m 2 all the residuals are < 1001tin. In o t h e r words we can say t h a t the 3 points are aligned within t,m" tracking precision. If the variance is > 10 -4 cm
CERN Detector R&D Collaboration RD-19 ~Hybrid silicon pirel telescope tested
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In such a way we select 11808 events (around the 94 % of our sample) fig.7a, 7b, and 7c show the correlations between the z coordinates of the three planes combined two by two, while fig.Td shows the z-coordinates in the middle plane versus the point where the line constructed using the z coordinates in the plane 1 and 3 intersects the middle detector. The wMth of the distributions can be explained by the angular spread of the partMes emerging from different intera.ction points of the target. We have studied the remaining 6 % in order to understand the nature of the so called "bad" canditates. We have subdivided this sample in two subsamples according to the value of the variance of the linear fit. In fact it, seems that this difference reflects a different interpretation of the data. 151 fig.8 we show the same coordinate correlation plots for candidates with a very bad variance of the linear fit(> 10 -e cm) and a D z > 3or (300 events,around 2 %). The interpretation of the data is that these are 2 track events not fully contained in our fiducial volume. As sketched in fig.9, the first track interceI)ts planes 2 an(l 3 this m e a n s t h a t one point is n~Jt ~m a s t r a i g h t line
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(but not 1), the second only plane 1. Those candMates are false single track events and should be removed from our sample when calculating efficiency. The second subsample contains candidates with a variance oil the linear fit in tile range 10 -4 < v a r < 10 -~ and a D z > 3or (600 events,around 4%). Those events seem to be low energy particles (like protons around 1 GeV ) which i)resent a big scattering in the middle plane. In this case, in fact, the correlation plots (figl0) show a good correlation between planes 1 and 2 and between planes 2 and 3 but a missing correlation between planes 1 and 3 because of the large scattering angle. The fraction of events found is consistent with the analytical evaluation of the probability that a proton of 1 GeV energy has a scattering in the middle plane leading to a deviation of 100/tin or more.
266
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shows a d i s t r i b u t i o n of this
b e a m axis i n t e r c e p t (in t h e t a r g e t r e g i o n ) , to be c o l n p a r e d w i t h f i g . l l b , where a n e n l a r g e d view of the corresl)ortding d i s t r i b u t i o n t a k e n Fig. 9. Schematic view in the x-z plane ,)f one event with one cluster /I)lane generated I)y two tracks.
from
fig.5 is s h o w n . T h e two s h a p e s are very s i m i l a r ( even if t h e n u I n b e r of e n t r i e s is q u i t e different). If we call " N e m p t y " t h e n u m b e r of e v e n t s
T h e efficiency is t h e l ) r o b a b i l i t y t h a t a t r a c k t r a v e r s i n g a pixel d e t e c t o r
will
a c t u a l l y be de-
t e c t e d . A n e s t i m a t e of t h e efficiency for p l a n e 2 was m a d e by a p p l y i n g a s i m i l a r c r i t e r i o n as
in t h e s a m p l e w i t h n o hits in t h e m i d d l e p l a n e a n d " N f u l l " t h e m u n b e r of e v e n t s in t h e s a m p l e w i t h one c l u s t e r per p l a n e , t h e efficiency for the m i d d l e l)lane is defined a,s:
for " g o o d " c a n d i d a t e s , i.e. o n e c l u s t e r in p l a n e 1 a n d 3, a n d n o r e q u i r e m e n t for the m i d d l e p l a n e . M o r e o v e r , a c a n d i d a t e is a c c e p t e d if t h e s t r a i g h t
E f f i c i e m c y = N f ull+ N/,,u N e m p t y = 99.2%
CERN Detector R&D Collaboration RD-19 /Hybrid silicon pixel telescope tested
267
References [1] P.E.Karchin, Use of pixel detectors in eleinentary particle iA~ysics,Proc.2ml.Int. Workshop on silicon pixel detect.rs,Leuven 1990 ,Nucl.Instr, Meth. A305 (1991 )497. [2] P.Jarron, Fast silicon detector systems for high luminosity ha
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[3] M.Camphell et al., A 10 MHz micropower CMOS tY, nt end for direct rea(hmt of pixel detectm's,Nucl.Instr, and Meth. A290(1990) 149. [4] W.Beusch et al., C E R N / D R D C 90-81,P22(RD19),
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Fig. 11. a) Distrilmtiml of the I,eam axis intel'cept f~,r events with one cluster/plane, b) Distribution . f the beam axis intercept f~r events with one cluster in the planes 1 and 3 and m~ hits in the middle plane, hi hoth cases a gaussian shape is used t~ fit the
RD Prolmsal flu" the devel~pment of hybrM and m.m,lithic silicon micropattern detectors, CERN,Cleneve,Sp.kesman E.H.M. Heijne [5] F. Anghimdfi et al., C E R N / E C P 91-26 ;A 1006 element hybrid silicon pixel detector with strobed binary output:presented by Erik.H.M. Heijne at the IEEE Nucl.Science Symposium 1991 5-9 November 1991,Santa Fe,USA to be published on IEEE.Trans.NucI.Sc Ns-39(1992) [[;] F. Anghin.lfi et al., CERN E C P / 9 2 - 6 Design and Performances . f the OMEGA-ION hybrid silicon pixel detector:presented by M.(?ampbell at the 6th EIIl'Ol)eall S y l l l p o S i l l l l | Oll S e n l i c < m d u c t o r
24-2~; Fehruary 1992,Milano,Italy
6. Conclusions For the first time a test of a hybrid silicon pixel detector wms carried out succesfully in a fixed target exI)eriment environment. It w~Ls possible to develoI.) a very simple and thst pattern recognition, without ambiguities ill space, for tile study of single and multitrack events. Tile test
has shown the good position accuracy of the pixel detector,whic, h was 25 ibm in the direction of the 75 # m pitch. The etficiency was measured to be 99.2%.
Detector