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dχ method using the shape analysis of the signal
Nuclex lnst~ments and Methods m Physics Research A 364 ( 1995) 2.58-264
=t!i!E!Y
ELSEVIER
Experimentalstudyof particle separationin a LKr ionization chamberby dIZ’/dx methodusing the shapeanalysisof the signal P. Cantoni a, L. Stagni a, PA. Kulinich b, C. Brazzelli ‘, F. Lanni ‘, B. Maggi ‘, F. Palomboc**, P.F.Manfredi d, V. Red, V. Speziali d
Received 6 Fcbruari 1995; revised form rrxckd
I I April 199.5
Abstract Charged particle (s-K) separation in the momentum range 0.5-0.7 GeV/c using a new method of shape analysis of the signal from a liquid krypton ( LKr) ionization chamber has been experimentally studied. The detector has been exposed to pions and protons at the Tl I test beam at the CERN PS. The shape of the preamplifier output signal was recorded by a waveform digitizer, then it was doubly “differentiated” in order to obtain few dE/ dx measurements inside a 2 cm LKr gap. Results on particle separation at three different energies are presented.
1.1. Beam selection
The KEDR LKr e.m. calorimeter consists of a set of ionization chambers operating in electron-pulse mode and read out by low-noise charge-sensitive preamplifiers [ I J. The pulse height information from the first consecutivechambers can be used for dE/ dx charged particle (r/K) separation in the momentum range OS-O.9 GeV/c. The total thickness of the liquid medium (4 cm of LKr for each chamber) however limits the number of chamber that can be used. due to shower development and nuclear interactions. We are investigating the possibility of a/K separation by the dE/ dx method by using only one (tire first) LKr ionization chamber. A LKr ionization chamber has been exposed to pions and protons at the TI I test-beam of the CERN PS. The signal out from the preamplifier connected to one pad channel was sent both to a peak-sensitive ADC and to a waveform digitizer. In this paper we present the results of particle sep aration by using the shape of the signal. Monte Carlo study of r/K separation by using this metbod in a LKr ionization chamber can be found elsewhere [2]. Results of the ADC analysis of the experimental data for particle separation have been already reported [ 31 and preliminary results on particb separation with a “current” analysis based on the maximum likelihood method can be found elsewhere [4].
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For the experimental study of rr/K separation in the momentum range 0.5-0.9 GeV/c. one has the difficult task of finding a kaon beam of such momenta with acceptable partitle rate. For this reason we have used a proton beam instead of the kaon beam. The proton beam momentum was selected in order to have proton velocity equal to the kaon veiocity at the required momentum. Such a proton produces the same ionization toss as the kaon with a momentum equal to that of the pion beam [ 5 1. The energy loss in the material along the beam was taken into account in order to obtain the necessary particle momentum at the entrance of tbe LKr chamber. We have exposed a prototype of LKr detector to pions and protons at the Tl I beam of the CERN PS. The particle contents of this beam can be chosen, but it is energy dependent and it has a significant amount of positrons in the case of pion beam (in particular at low energy) and of pions in the case of the proton beam. So one has to make an independent beam particle selection. For rejection of the positrons in the pion beam and of the pions in the proton beam a gas Cherenkov counter and time-of-flight measurements were Used.
The pion beam in the momentum range OS-O.9 t&V/c has a significant contamination of muons, too, because of pion decay. This problem was studied by Monte Carlo simulation of our test setup. It was shown that the contamination of muons in the pion beam, which were able to produce a
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trigger signal, is a b o u t 2 % . T h e s e m u o n s m u o n ionization spectrum d o e s not practically differ from the p i o n o n e . W e h a v e u s e d a b e a m rate a r o u n d a t h o u s a n d particles p e r s e c o n d in o r d e r to e x c l u d e d y n a m i c saturation of the preamplifiers. 2. E x p e r i m e n t a l setup T h e experimental setup (Fig. I ) consisted of a b e a m defining system a n d a L K r detector. T h e b e a m w a s d e f i n e d b y four scintillation counters a n d a C h e r e n k o v g a s c o u n t e r C. T h e anticoincidence c o u n t e r & ( 1 8 x 3 0 cm*) h a d a S X S cm* w i n d o w a r o u n dthe b e a m . T h r e e scintillation c o u n ters S t. S2, $ 1 (in coincidence) a n d C?,A t (in anticoincid e n c e ) w e r e u s e d for g e n e r a t i n ga S Iart signal of a fast p r o g r a m m a b l e trigger unit [ 61. ‘B v o scintillation counters S r a n d S > ,at a relative distance of S .95 m. with d o u b l e sides readout, w e r e u s e d for timeof-flight ( T O F ) m e a s u r e m e n t s .T h e T G F information a n d the a m p l i t u d e of the s e c o n d T O F c o u n t e r ( S 3 ) signal w e r e u s e d in a fast selection of b e a m particles. T h e u s e of a n anticoincidencecountera m u n d the b e a m b e h i n d the detector a n d of the a m p l i t u d e information from the T O F c o u n t e r ( Sj ) allow the suppressionof multipatticle eventsoriginated from b e a m particle interaction. T h e S tarr signal s y n c h r o n i z e d b y the m e a n - t i m e d signal of the first T G F c o u n t e r (St ), a n d the m e a n - t i m e d signal from the s e c o n d T O F c o u n t e r w e r e sent to the first c h a n n e l of a m e m o r y look u p ( M L U ) unit 1 6 1 for fast TCIOFm e a surement. T h e s e c o n d c h a n n e l of the M L U w a s u s e d a s a fast Q D C of a linear s u m of signals from b o t h p h o t o m u h i pliers (EMI 9 9 5 4 - K B - 0 3 ) of the s e c o n dT O F counter. T h e M L U is a p m g r a m m a b i e C A M A C m o d u l e for fast t w o d i m e n s i o n a l ( 3 2 x 3 2 ) analysis. It h a s two fast S-bit Q B C a n d a R A M s c h e m e .T h e m e m o r y contentsc a n b e c o n tmlkd via a C A M A C bus. T h e a d d r e s sof the m e m o r y cell d u r i n g the d a t a taking is d e t e t m i n e d b y the c o d e s at the output of b o t h Q D C . T h e M L U g e n e r a t e sa signal w h e n e v e rthe input ‘IDF signals a n d the p u l s e height of the a m & g s u m signal fmm the s e c o n d 1 D F c o u n t e r c o r r e s p o n d to the n q u e s t e d particle ( p i o n o r p r o t o n in o u r case). This s c h e m e p r o v i d e s a fast ( l o o ns) trigger a n d permits palticie sekcth b y c h a n g i n g “tim e ” position of a mask. For mjection of muhipartick events inside the “w i n d o w ” of the anticoinci-
d e n c e c o u n t e r the M L U unit w a s p r o g r a m m e d to s u p p r e s s events with a l a r g e a m p l i t u d e of the signal in the S J counter. F o u r output signals after the T O F constant fraction discriminators w e r e sent to a T D C ( L e - C r o y 2 2 2 8 A ) for offline analysis. T h e g a s C h e r e n k o v c o u n t e r a v a i l a b f eo n theTl1 b e a m w a s u s e d to veto positrons w h o s e contents in the b e a m rapidly increaseswith d e c r e a s i n gm o m e n t u m of the p i o n b e a m . Its efficiency is not k n o w n exactly, but taking into a c c o u n t the spectrum of the signals a n d the threshold of the discrimin a t o r w e c o n c l u d e that it s h o u l d b e h i g h e r t h a n 9 0 % . B e sides that, positrons w e r e additionally s u p p r e s s e db y m o r e t h a n S radiation lengths of material with the anticoincidence c o u n t e r A I a n d the s e c o n d T O F c o u n t e r a m p l i t u d e rejection. R e s i d u a l positrons c o u l d b e s e p a r a t e din part b y T G F offline analysis. A t 0.7 G e V /c. for e x a m p l e , a p i o n h a s a d e l a y of a b o u t 0.4 n s (in o u r setup g e o m e t r y ) in c o m p a r i s o n to a relativistic positron with a m e a s u r e dtime resolution of o u r T G F system of 0 . 2 3 n s for the p i o n b e a m .
2.1. L K r detector
T h e L K r detector, consisting of a set of 8 x I2 cm* ionization c h a m b e r swith a 2 c m g a p , w a s inserted into a ctyostat of two coaxial stainless steel ( 3 l6L) vessels.T h e i n n e r o n e h a d a geometrical v o l u m e of 6.7 I. T h e c h a m b e r s w e m p l a c e d o r t h o g o n a l to the b e a m axis. T h e c h a m b e r e k z t m d e s weFemadeofFR4(0.Smmthick).coveredonbothsides with 1 8 p m c o p p e r layers. In total t h e r e w e r e n i n e eiect&es alternatively corutected to g r o u n d (5) a n d to h i g h voltage (4). T h e first a n d the last signal ekztmdes with respect to the b e a m h a d p a d s of 8 x 8 cm*, white the o t h e r t h r e e sign a l ekctrodes w e r e divided into 1 0 m m w i d e strips in the x a n d y dimctions. O n e strip p l a n e h a d d o u b k - s i d e o & o g o n a l strips. A d e h i n plastic s u p p o r t w a s u s e d for the e l u % m i e s inside the cryostat. B e f o m liquefaction of K r the c h a m b e r wasbakedoutfor2daysatabout60°c. W e u s e d n o purification system for g a s e o u skrypton. T h e iudustrial g a s at o u r disposat h a d e R c u o n e @ v e impwities b e l o w I p p m . G a s e o u skrypton flowed into the d a e c c o r t h n w g h a h i g h purity stainless steef L i n eat a p r e s s u r ea b o u t 1.6~~w~~i~fi~by~s~l~~ in a h e a t - e x c h a n g etube. T o check the level of LiCr d a & g the lquefaction, the effect of i n c r e a s eof the inter&ectmde
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capacitance due to dielectric constant of LKr was used. Voltage pulses were fed to the anodes and the associated preamplitier signals were monitored while filling the detector. The temperature of liquid krypton was kept during the test at (- 147 f I .S)‘C and continuously monitored by 3 platinum resistor thermometers placed on the electrodes’ support at different heights. The five central strips in each plane and pads wete connected to individual integrating preamplifiers based on the FET SNJ903L. The equivalent noise charge measured in this test was about 595 electrons with a shaper time constant 7ti = 0.5 ps. The preamplifier output signals were sent to shaper-amplifiers with semi-gaussian shaping ( 7,t, = 500 ns). The signals were digitized by a peak sensitive ADC (SILENA Mod. 4418/V) and then sent to a dedicated personal computer. The other parts of the signal elect&es were grounded. A shielded preamplitier box was mounted on the top flange of the cryostat. The preamplifiers wete dc coupled to electrodes, each having a nominal feedback capacitance of I pF. For chatge calibration. a voltage pulse fmtn a precision pulse generator was injected into the pmamplifien through known test capacitots after each beam spili during the run. Only the last ionization chamber (2 cm LKr gap) pad signal was used for the ahape analysis presented in thii paper. The signal from the preamplifier of this pad channel was also sent after a wide band amplifier to a waveform digitizer (Lc-Caq Mod. 2252) with 40 MHz sampbng frequency. This recorder has a 316 sample recotd length and .t 10 bit ~~iu~n[7~.So#Feiacorded~shapeof~padsigrul within a 7.9 w time ‘interval. To reduce high frequency ripple in tbe high voltage a pair of passive tow-pass filters were used.
Data were taken for pions at momenta (580. 680 and 780 MeVlc) and for ptotons at 1100, 1270 and 1440 MeV/c. Taking into account the energy loss of the particles. the beam momenta were set so as to have pions and “kaons” at the entrance of the last ionization chambers with momenta of 500,600 and 700 MeV/c. respectively. The events used in the analysis were selected according to the following OFF-LINE criteria: ti) a cut based on the TylF analysis (meanf0.4 ns); (ii) a cut based on the second TOF counter signal amplitude analysis; (iii) cuts on the pulse height of the total strip plane chatge. and on (x. y) coordinates of the particle in the strip planes to reject multiparticle events, and to exclude current edge effect 12 I; (iv) cuts on the parameters of linear fit at the plateau before the signal, which is seen in Fig. 2. These cuts reject events whiih are affected by external pickup noise or contain the tail of the previous signal; (VI a cut based on the shape analysis of the signal itself in order to reject signals with distinguishable external noise influence. A total statistics of 2000 events survived these cuts and was used for further analysis.
4. Data aualyaia The digitizer information of tke selected ‘*kaon” events at 0.5 GeV/c was analyzed to tin the valuesof the drift time ( Tpl ) and t%e electrou lifetime (I~). This sample of events was chosen for such an analysis because of the smallest influence of energetic &electrons on the signal shape. The current induced in the gap is given by the formula: I(r) = Id I - r/T&) exp(
for 0 I I 5 Tpr.
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By integration of this expression we obtain the chatge Q( r ):
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The distribution of the other fitting parameter re (Fig. 3a) is rather broad. This can be expected because the formula used in the fit. For thk reason the experimental value of 7e has been estimated by the analysis of the decmase of the mean values in different quasi-gaps (see later). of d&/ dx( i), Data taken at different high voltages have been used to study tbe electric fieM dependenceof the pulse height which wasfittedbytheformula (81:
where C, is the scale factor and E+ is the recombination constant. The value obtained for the fitted parameter G is (0.28 f 0.08) kV/cm. (Fig. 4). Here it is not taken into
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account the correction for *he “ballistic deficit” IS] on the shaper signal amplitude. 4. I. Shape analysis All data for shape analysis were taken at the high voltage 3.5 kV. The signal shape analysis of the events was done in the following way. The set of wave mcotder processing channels (starting at channel No. 45 in Fig 2 was divided into a few gtoups ( Ns, = 5-8),andthesumofthechannels in each group was used for the subsequent analysis. As a result of numerical differentiation of the grouped channels. asetof(N, - I ) values of induced “cument” was obtained.
Rrs. A 364 (1995) ZSX-264
262
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Fig. 6 (a) Height pulse speclra from AX for die pion and “kaon” beams at 0.6 C&V/c (the digitizer signal spectra are sirnib); (b) dE/dr,, sxkcuon derribed in Ihc Ext. &is (E’,,,): threshold al which lhc left til of “kaon” sp-cmtm contains 15.83% of these pa-tick events.
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60 dE/dx,
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Fig. 7. Normalization using eumdaiive dislsiln~tion funelioa for dr “inon” al 0.6 f&V/c. dEj dx( I ) IMU (a) and dE/dt(d)ltrsr WCI%~S: CWUU~&C dihb~rio~ funetiow fw the furc (c) and fowtb (d) quasi-gaps. ‘llte anuws show the sequence of u&g bjv
(b) ~pceoa for sebzcwd in &is w
0.5
0.6
Fig. 8. (a-K) panicle separation (FQOcb) using Ihe 2 (Fig. 8a) or 3 (fig. for differem numbers of quasi-gaps.
A double numerical differentiation of the grouped channels gave ( Ng - 2) values of dE/ dx( i)corresponding to an actual number of quasi-gaps Nar = ( NBr - 2). Thespectntmof dE/ti(i),foranyi(thedistribution of ionization in the quasi-gap) differs from the AWJ pulse height spectrum because of the extension of the chamcteristic tail of the Landau distribution, though the electronic noise influence increases too. No measurable difference in the tail’s characteristic for these spectra was found. Because of the finite value of re the mean values of dE/dx( i)spectra for the sequence of quasi-gaps decrease at some rate. This feature was used to estimate the free electton lifetime. For NW = 4 the ratio of (dE/dx(n)-)/(
dE/dx(ll),,,&
0. 6
0.7 P {G&‘/c)
for n = 2.3.4
were used both for experimental data dud for Monte Carlo events with the same values of T+ and time duration of the groups. The spectrum of dE/dx( I)was not used because of possible effects of systematic errors. This influence can be due to the preamplifier or to some uncertainty in the stan of the first group. Fig. 5 shows the experimental and simulated results. From this analysis we concluded that the measured free electron lifetime is about 27 &s. Taking into account the calibration data. we found that the measured charge from the 2 cm LKrchamber was about 2.84 times smaller than expected from calculations using W = 20.5 eV/pair [I] and a free electron lifetime re = 27 @. We consider this effect as due to the recombination process. A similar rest& for non-puri8ed krypton was mentioned in ilO1. As a measufe of panicle separation, the percentgge of pions overlapped ( BCIO%) is used hemafter. It is equal to Ulcratioofthenumberofthcpsrtic~intheleft(pion)
8b) lcwes~ dE/drfc)&
in calculation of h- *an
0.7 P (GeV/c) values of the energy loss
spectrum with deposited energy larger than Ekrs (Fig. 6) to the total number of events in this spectrum. Et15is defined as the threshold value at which the left tail of the “kaon” energy spectrum contains 15.83% of the total number of “kaons’*. and it was determined for each particular case. This value is used because it is equal to half of the confidence level associated with a la deviation for the normal distribution. This measure of separation permits the analysis of spectra with a non-Gaussian shape. At comparatively small free electron lifetimes (Fig. 5) one should normalize the different N, dE/ dr( i), values in each event to cormct the systematic decrease of the “second derivative”. The responses of ail quasi-gaps inside a gap shot&J be identical in principle. A notmalization involving tbe cumulative disttibut.ion bmctions for each parobtained for all data samples ticular spectra dE/ dr( i), in our momentum range (OS-O.7 GeV/c) showed suitable result. Fromthe spectraof dE/dx(i), forall events (at a certain momentum in the following anatysis) (Figs. 7ab). the cumulative disttibution function for each quasi-gap was calculated (Figs. 7c.d). Fig. 7 depicts such a normalization to the first quasi-gap spectrum for the fourth quasi-gap. ta this case any systematie inthtence on the beginning of the signal shape is not so dangerous. The normalized values of dE/dx(i)&, for the generic event n ant different because of the accidental egpearancc of Sclectrons along the tmclt, the pmamplitier ekctrottie noise dismrbamz and the residual influence of the e%tSrtal pickup measurement. and noise.Themsolutionofthenmgyloss ~f~~~Of~~,US~~~Of themmcatedmean[12~,dependontbemtmberofsampka wasstudiedbyanaiyziythetnean of dEf dx(i)“, L3epaah(rn)
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Fig. 9. Experimemd (n-K) parUe separation (POCS) usingthe methodof Rexible rejection. The errors an: shuwn for ADC and Na curves. For olher awes te e,rrcm we roughly equal lo the curresponding values in the case of Nse = 4.
on the momentum of particles for Nsr = 3-6. A fixed number (2 and 3) of lowest dE/ dx( i)&$ values was used in the analysis. The curves are simple exponent fit iines. For mmparison !he pot)% for the ADC spectra is also pioned. Another method 121 of flexible rejection was studcti II~ the experimental data analysis. It uses different numbers of selected values of dE/dx( i)k- in calculation of the mean value dE/ dx,r for each event. The number N, of the lowest values of dE/dx(i)L used in the calculation of dE/ dx,r was determined with the formula: N,,,,,= NvL!IFIX
k,( N% - I )
dE/dx,
- dE/ dx,,,,” >* dEldxm,n
where dE/ dxlaal and dE/ dxmin are maximal and minimal valuesof dE/dx( i)&. The minimal number of quasi-gaps taken into account was limited to N, 2 I. A few values of L, ds is described in [ 21. were used for each value of the particle momentum. Then the optimal value of &, was determined by analyzing the results at all three particle momenta. Particle separation using the method of flexible rejection (Fig. 9) gives better results than those obtained with the method of truncated mean.
= 4 separaion
5. Conclusion The possibility of improving the separation of rr- and kmesons in a single gap of a liquid krypton uniform ionization chamber in the momentum range OS-O.7 GeV/c using the shape analysis of the signal has been experimentally studied. However, in our LKr chamber test-experiment the real signal was 2.84 times tess than the predicted one. This resulted in a degradation of the experimental separation capability compared to the calculated result. We considered it as a result of the use of non-purified krypton. A better experimental separation capability (in agreement with Monte Carlo simulation) could be obtained with an optimized setup. The front-end electronics used in this test, for example, was designed for high capacitance detector 1131. References ItI 121 13 I 141
V.M. AuldKnkoct al.. Nucl. Instr~md Me&. A 316 (1992) 8. PA.Kutinich.JtNR~.El19497,~l994tRu~nedtoNIMl. l? Cantoni a al., Nucl. Insu. end Meth. A 344 ( 1994) 156. P. Gmtoni et al.. particle ldentilicaticn in a LKr ionizatiun chasnber by umlttpte induced cuftxm measurement3 u&g the shslx mmlydr of the signal. Pnx. 6th FairsMeeting on advanced detecmrs, La Biodote l&u d’ERm+ tmly. May 22 1994. ~S~U.F~.Ann.Rev.Nltcl.SEi.l3(1%3)1. 16) PA. Kutiuicb. R Toledo. JMR Rep. PI087-l 12. Dubna, 1987. 171 LecmyReaemcbtasmunenmtionCaietog. 1992. (81 G. Jaffe. Ann. Phyr. 42 t 1913) 303. 191 J. R. Gipsale. t&cl. IIISR. and Me&. I I I t 1973) 345. 1 I01 E Aprik et al.. Nuci. tnsfr, and MC&. A 327 t 1993125. I I I I PK. Lebedev. V.I. Pryaatchntkov,Nucl. Insw. msd Meth. A 327 t 19931 222. II21 D.Jemmee~J..Nuct. 1ma.awJMeth.A III tl973)2W. I 131 WM. Aulchwkoet at., Nuol. Insus and Me& A 289 t 19%) 468.
=t!i!E!Y
ELSEVIER
Experimentalstudyof particle separationin a LKr ionization chamberby dIZ’/dx methodusing the shapeanalysisof the signal P. Cantoni a, L. Stagni a, PA. Kulinich b, C. Brazzelli ‘, F. Lanni ‘, B. Maggi ‘, F. Palomboc**, P.F.Manfredi d, V. Red, V. Speziali d
Received 6 Fcbruari 1995; revised form rrxckd
I I April 199.5
Abstract Charged particle (s-K) separation in the momentum range 0.5-0.7 GeV/c using a new method of shape analysis of the signal from a liquid krypton ( LKr) ionization chamber has been experimentally studied. The detector has been exposed to pions and protons at the Tl I test beam at the CERN PS. The shape of the preamplifier output signal was recorded by a waveform digitizer, then it was doubly “differentiated” in order to obtain few dE/ dx measurements inside a 2 cm LKr gap. Results on particle separation at three different energies are presented.
1.1. Beam selection
The KEDR LKr e.m. calorimeter consists of a set of ionization chambers operating in electron-pulse mode and read out by low-noise charge-sensitive preamplifiers [ I J. The pulse height information from the first consecutivechambers can be used for dE/ dx charged particle (r/K) separation in the momentum range OS-O.9 GeV/c. The total thickness of the liquid medium (4 cm of LKr for each chamber) however limits the number of chamber that can be used. due to shower development and nuclear interactions. We are investigating the possibility of a/K separation by the dE/ dx method by using only one (tire first) LKr ionization chamber. A LKr ionization chamber has been exposed to pions and protons at the TI I test-beam of the CERN PS. The signal out from the preamplifier connected to one pad channel was sent both to a peak-sensitive ADC and to a waveform digitizer. In this paper we present the results of particle sep aration by using the shape of the signal. Monte Carlo study of r/K separation by using this metbod in a LKr ionization chamber can be found elsewhere [2]. Results of the ADC analysis of the experimental data for particle separation have been already reported [ 31 and preliminary results on particb separation with a “current” analysis based on the maximum likelihood method can be found elsewhere [4].
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For the experimental study of rr/K separation in the momentum range 0.5-0.9 GeV/c. one has the difficult task of finding a kaon beam of such momenta with acceptable partitle rate. For this reason we have used a proton beam instead of the kaon beam. The proton beam momentum was selected in order to have proton velocity equal to the kaon veiocity at the required momentum. Such a proton produces the same ionization toss as the kaon with a momentum equal to that of the pion beam [ 5 1. The energy loss in the material along the beam was taken into account in order to obtain the necessary particle momentum at the entrance of tbe LKr chamber. We have exposed a prototype of LKr detector to pions and protons at the Tl I beam of the CERN PS. The particle contents of this beam can be chosen, but it is energy dependent and it has a significant amount of positrons in the case of pion beam (in particular at low energy) and of pions in the case of the proton beam. So one has to make an independent beam particle selection. For rejection of the positrons in the pion beam and of the pions in the proton beam a gas Cherenkov counter and time-of-flight measurements were Used.
The pion beam in the momentum range OS-O.9 t&V/c has a significant contamination of muons, too, because of pion decay. This problem was studied by Monte Carlo simulation of our test setup. It was shown that the contamination of muons in the pion beam, which were able to produce a
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trigger signal, is a b o u t 2 % . T h e s e m u o n s m u o n ionization spectrum d o e s not practically differ from the p i o n o n e . W e h a v e u s e d a b e a m rate a r o u n d a t h o u s a n d particles p e r s e c o n d in o r d e r to e x c l u d e d y n a m i c saturation of the preamplifiers. 2. E x p e r i m e n t a l setup T h e experimental setup (Fig. I ) consisted of a b e a m defining system a n d a L K r detector. T h e b e a m w a s d e f i n e d b y four scintillation counters a n d a C h e r e n k o v g a s c o u n t e r C. T h e anticoincidence c o u n t e r & ( 1 8 x 3 0 cm*) h a d a S X S cm* w i n d o w a r o u n dthe b e a m . T h r e e scintillation c o u n ters S t. S2, $ 1 (in coincidence) a n d C?,A t (in anticoincid e n c e ) w e r e u s e d for g e n e r a t i n ga S Iart signal of a fast p r o g r a m m a b l e trigger unit [ 61. ‘B v o scintillation counters S r a n d S > ,at a relative distance of S .95 m. with d o u b l e sides readout, w e r e u s e d for timeof-flight ( T O F ) m e a s u r e m e n t s .T h e T G F information a n d the a m p l i t u d e of the s e c o n d T O F c o u n t e r ( S 3 ) signal w e r e u s e d in a fast selection of b e a m particles. T h e u s e of a n anticoincidencecountera m u n d the b e a m b e h i n d the detector a n d of the a m p l i t u d e information from the T O F c o u n t e r ( Sj ) allow the suppressionof multipatticle eventsoriginated from b e a m particle interaction. T h e S tarr signal s y n c h r o n i z e d b y the m e a n - t i m e d signal of the first T G F c o u n t e r (St ), a n d the m e a n - t i m e d signal from the s e c o n d T O F c o u n t e r w e r e sent to the first c h a n n e l of a m e m o r y look u p ( M L U ) unit 1 6 1 for fast TCIOFm e a surement. T h e s e c o n d c h a n n e l of the M L U w a s u s e d a s a fast Q D C of a linear s u m of signals from b o t h p h o t o m u h i pliers (EMI 9 9 5 4 - K B - 0 3 ) of the s e c o n dT O F counter. T h e M L U is a p m g r a m m a b i e C A M A C m o d u l e for fast t w o d i m e n s i o n a l ( 3 2 x 3 2 ) analysis. It h a s two fast S-bit Q B C a n d a R A M s c h e m e .T h e m e m o r y contentsc a n b e c o n tmlkd via a C A M A C bus. T h e a d d r e s sof the m e m o r y cell d u r i n g the d a t a taking is d e t e t m i n e d b y the c o d e s at the output of b o t h Q D C . T h e M L U g e n e r a t e sa signal w h e n e v e rthe input ‘IDF signals a n d the p u l s e height of the a m & g s u m signal fmm the s e c o n d 1 D F c o u n t e r c o r r e s p o n d to the n q u e s t e d particle ( p i o n o r p r o t o n in o u r case). This s c h e m e p r o v i d e s a fast ( l o o ns) trigger a n d permits palticie sekcth b y c h a n g i n g “tim e ” position of a mask. For mjection of muhipartick events inside the “w i n d o w ” of the anticoinci-
d e n c e c o u n t e r the M L U unit w a s p r o g r a m m e d to s u p p r e s s events with a l a r g e a m p l i t u d e of the signal in the S J counter. F o u r output signals after the T O F constant fraction discriminators w e r e sent to a T D C ( L e - C r o y 2 2 2 8 A ) for offline analysis. T h e g a s C h e r e n k o v c o u n t e r a v a i l a b f eo n theTl1 b e a m w a s u s e d to veto positrons w h o s e contents in the b e a m rapidly increaseswith d e c r e a s i n gm o m e n t u m of the p i o n b e a m . Its efficiency is not k n o w n exactly, but taking into a c c o u n t the spectrum of the signals a n d the threshold of the discrimin a t o r w e c o n c l u d e that it s h o u l d b e h i g h e r t h a n 9 0 % . B e sides that, positrons w e r e additionally s u p p r e s s e db y m o r e t h a n S radiation lengths of material with the anticoincidence c o u n t e r A I a n d the s e c o n d T O F c o u n t e r a m p l i t u d e rejection. R e s i d u a l positrons c o u l d b e s e p a r a t e din part b y T G F offline analysis. A t 0.7 G e V /c. for e x a m p l e , a p i o n h a s a d e l a y of a b o u t 0.4 n s (in o u r setup g e o m e t r y ) in c o m p a r i s o n to a relativistic positron with a m e a s u r e dtime resolution of o u r T G F system of 0 . 2 3 n s for the p i o n b e a m .
2.1. L K r detector
T h e L K r detector, consisting of a set of 8 x I2 cm* ionization c h a m b e r swith a 2 c m g a p , w a s inserted into a ctyostat of two coaxial stainless steel ( 3 l6L) vessels.T h e i n n e r o n e h a d a geometrical v o l u m e of 6.7 I. T h e c h a m b e r s w e m p l a c e d o r t h o g o n a l to the b e a m axis. T h e c h a m b e r e k z t m d e s weFemadeofFR4(0.Smmthick).coveredonbothsides with 1 8 p m c o p p e r layers. In total t h e r e w e r e n i n e eiect&es alternatively corutected to g r o u n d (5) a n d to h i g h voltage (4). T h e first a n d the last signal ekztmdes with respect to the b e a m h a d p a d s of 8 x 8 cm*, white the o t h e r t h r e e sign a l ekctrodes w e r e divided into 1 0 m m w i d e strips in the x a n d y dimctions. O n e strip p l a n e h a d d o u b k - s i d e o & o g o n a l strips. A d e h i n plastic s u p p o r t w a s u s e d for the e l u % m i e s inside the cryostat. B e f o m liquefaction of K r the c h a m b e r wasbakedoutfor2daysatabout60°c. W e u s e d n o purification system for g a s e o u skrypton. T h e iudustrial g a s at o u r disposat h a d e R c u o n e @ v e impwities b e l o w I p p m . G a s e o u skrypton flowed into the d a e c c o r t h n w g h a h i g h purity stainless steef L i n eat a p r e s s u r ea b o u t 1.6~~w~~i~fi~by~s~l~~ in a h e a t - e x c h a n g etube. T o check the level of LiCr d a & g the lquefaction, the effect of i n c r e a s eof the inter&ectmde
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capacitance due to dielectric constant of LKr was used. Voltage pulses were fed to the anodes and the associated preamplitier signals were monitored while filling the detector. The temperature of liquid krypton was kept during the test at (- 147 f I .S)‘C and continuously monitored by 3 platinum resistor thermometers placed on the electrodes’ support at different heights. The five central strips in each plane and pads wete connected to individual integrating preamplifiers based on the FET SNJ903L. The equivalent noise charge measured in this test was about 595 electrons with a shaper time constant 7ti = 0.5 ps. The preamplifier output signals were sent to shaper-amplifiers with semi-gaussian shaping ( 7,t, = 500 ns). The signals were digitized by a peak sensitive ADC (SILENA Mod. 4418/V) and then sent to a dedicated personal computer. The other parts of the signal elect&es were grounded. A shielded preamplitier box was mounted on the top flange of the cryostat. The preamplifiers wete dc coupled to electrodes, each having a nominal feedback capacitance of I pF. For chatge calibration. a voltage pulse fmtn a precision pulse generator was injected into the pmamplifien through known test capacitots after each beam spili during the run. Only the last ionization chamber (2 cm LKr gap) pad signal was used for the ahape analysis presented in thii paper. The signal from the preamplifier of this pad channel was also sent after a wide band amplifier to a waveform digitizer (Lc-Caq Mod. 2252) with 40 MHz sampbng frequency. This recorder has a 316 sample recotd length and .t 10 bit ~~iu~n[7~.So#Feiacorded~shapeof~padsigrul within a 7.9 w time ‘interval. To reduce high frequency ripple in tbe high voltage a pair of passive tow-pass filters were used.
Data were taken for pions at momenta (580. 680 and 780 MeVlc) and for ptotons at 1100, 1270 and 1440 MeV/c. Taking into account the energy loss of the particles. the beam momenta were set so as to have pions and “kaons” at the entrance of the last ionization chambers with momenta of 500,600 and 700 MeV/c. respectively. The events used in the analysis were selected according to the following OFF-LINE criteria: ti) a cut based on the TylF analysis (meanf0.4 ns); (ii) a cut based on the second TOF counter signal amplitude analysis; (iii) cuts on the pulse height of the total strip plane chatge. and on (x. y) coordinates of the particle in the strip planes to reject multiparticle events, and to exclude current edge effect 12 I; (iv) cuts on the parameters of linear fit at the plateau before the signal, which is seen in Fig. 2. These cuts reject events whiih are affected by external pickup noise or contain the tail of the previous signal; (VI a cut based on the shape analysis of the signal itself in order to reject signals with distinguishable external noise influence. A total statistics of 2000 events survived these cuts and was used for further analysis.
4. Data aualyaia The digitizer information of tke selected ‘*kaon” events at 0.5 GeV/c was analyzed to tin the valuesof the drift time ( Tpl ) and t%e electrou lifetime (I~). This sample of events was chosen for such an analysis because of the smallest influence of energetic &electrons on the signal shape. The current induced in the gap is given by the formula: I(r) = Id I - r/T&) exp(
for 0 I I 5 Tpr.
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By integration of this expression we obtain the chatge Q( r ):
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The distribution of the other fitting parameter re (Fig. 3a) is rather broad. This can be expected because the formula used in the fit. For thk reason the experimental value of 7e has been estimated by the analysis of the decmase of the mean values in different quasi-gaps (see later). of d&/ dx( i), Data taken at different high voltages have been used to study tbe electric fieM dependenceof the pulse height which wasfittedbytheformula (81:
where C, is the scale factor and E+ is the recombination constant. The value obtained for the fitted parameter G is (0.28 f 0.08) kV/cm. (Fig. 4). Here it is not taken into
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account the correction for *he “ballistic deficit” IS] on the shaper signal amplitude. 4. I. Shape analysis All data for shape analysis were taken at the high voltage 3.5 kV. The signal shape analysis of the events was done in the following way. The set of wave mcotder processing channels (starting at channel No. 45 in Fig 2 was divided into a few gtoups ( Ns, = 5-8),andthesumofthechannels in each group was used for the subsequent analysis. As a result of numerical differentiation of the grouped channels. asetof(N, - I ) values of induced “cument” was obtained.
Rrs. A 364 (1995) ZSX-264
262
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Fig. 7. Normalization using eumdaiive dislsiln~tion funelioa for dr “inon” al 0.6 f&V/c. dEj dx( I ) IMU (a) and dE/dt(d)ltrsr WCI%~S: CWUU~&C dihb~rio~ funetiow fw the furc (c) and fowtb (d) quasi-gaps. ‘llte anuws show the sequence of u&g bjv
(b) ~pceoa for sebzcwd in &is w
0.5
0.6
Fig. 8. (a-K) panicle separation (FQOcb) using Ihe 2 (Fig. 8a) or 3 (fig. for differem numbers of quasi-gaps.
A double numerical differentiation of the grouped channels gave ( Ng - 2) values of dE/ dx( i)corresponding to an actual number of quasi-gaps Nar = ( NBr - 2). Thespectntmof dE/ti(i),foranyi(thedistribution of ionization in the quasi-gap) differs from the AWJ pulse height spectrum because of the extension of the chamcteristic tail of the Landau distribution, though the electronic noise influence increases too. No measurable difference in the tail’s characteristic for these spectra was found. Because of the finite value of re the mean values of dE/dx( i)spectra for the sequence of quasi-gaps decrease at some rate. This feature was used to estimate the free electton lifetime. For NW = 4 the ratio of (dE/dx(n)-)/(
dE/dx(ll),,,&
0. 6
0.7 P {G&‘/c)
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were used both for experimental data dud for Monte Carlo events with the same values of T+ and time duration of the groups. The spectrum of dE/dx( I)was not used because of possible effects of systematic errors. This influence can be due to the preamplifier or to some uncertainty in the stan of the first group. Fig. 5 shows the experimental and simulated results. From this analysis we concluded that the measured free electron lifetime is about 27 &s. Taking into account the calibration data. we found that the measured charge from the 2 cm LKrchamber was about 2.84 times smaller than expected from calculations using W = 20.5 eV/pair [I] and a free electron lifetime re = 27 @. We consider this effect as due to the recombination process. A similar rest& for non-puri8ed krypton was mentioned in ilO1. As a measufe of panicle separation, the percentgge of pions overlapped ( BCIO%) is used hemafter. It is equal to Ulcratioofthenumberofthcpsrtic~intheleft(pion)
8b) lcwes~ dE/drfc)&
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0.7 P (GeV/c) values of the energy loss
spectrum with deposited energy larger than Ekrs (Fig. 6) to the total number of events in this spectrum. Et15is defined as the threshold value at which the left tail of the “kaon” energy spectrum contains 15.83% of the total number of “kaons’*. and it was determined for each particular case. This value is used because it is equal to half of the confidence level associated with a la deviation for the normal distribution. This measure of separation permits the analysis of spectra with a non-Gaussian shape. At comparatively small free electron lifetimes (Fig. 5) one should normalize the different N, dE/ dr( i), values in each event to cormct the systematic decrease of the “second derivative”. The responses of ail quasi-gaps inside a gap shot&J be identical in principle. A notmalization involving tbe cumulative disttibut.ion bmctions for each parobtained for all data samples ticular spectra dE/ dr( i), in our momentum range (OS-O.7 GeV/c) showed suitable result. Fromthe spectraof dE/dx(i), forall events (at a certain momentum in the following anatysis) (Figs. 7ab). the cumulative disttibution function for each quasi-gap was calculated (Figs. 7c.d). Fig. 7 depicts such a normalization to the first quasi-gap spectrum for the fourth quasi-gap. ta this case any systematie inthtence on the beginning of the signal shape is not so dangerous. The normalized values of dE/dx(i)&, for the generic event n ant different because of the accidental egpearancc of Sclectrons along the tmclt, the pmamplitier ekctrottie noise dismrbamz and the residual influence of the e%tSrtal pickup measurement. and noise.Themsolutionofthenmgyloss ~f~~~Of~~,US~~~Of themmcatedmean[12~,dependontbemtmberofsampka wasstudiedbyanaiyziythetnean of dEf dx(i)“, L3epaah(rn)
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Fig. 9. Experimemd (n-K) parUe separation (POCS) usingthe methodof Rexible rejection. The errors an: shuwn for ADC and Na curves. For olher awes te e,rrcm we roughly equal lo the curresponding values in the case of Nse = 4.
on the momentum of particles for Nsr = 3-6. A fixed number (2 and 3) of lowest dE/ dx( i)&$ values was used in the analysis. The curves are simple exponent fit iines. For mmparison !he pot)% for the ADC spectra is also pioned. Another method 121 of flexible rejection was studcti II~ the experimental data analysis. It uses different numbers of selected values of dE/dx( i)k- in calculation of the mean value dE/ dx,r for each event. The number N, of the lowest values of dE/dx(i)L used in the calculation of dE/ dx,r was determined with the formula: N,,,,,= NvL!IFIX
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- dE/ dx,,,,” >* dEldxm,n
where dE/ dxlaal and dE/ dxmin are maximal and minimal valuesof dE/dx( i)&. The minimal number of quasi-gaps taken into account was limited to N, 2 I. A few values of L, ds is described in [ 21. were used for each value of the particle momentum. Then the optimal value of &, was determined by analyzing the results at all three particle momenta. Particle separation using the method of flexible rejection (Fig. 9) gives better results than those obtained with the method of truncated mean.
= 4 separaion
5. Conclusion The possibility of improving the separation of rr- and kmesons in a single gap of a liquid krypton uniform ionization chamber in the momentum range OS-O.7 GeV/c using the shape analysis of the signal has been experimentally studied. However, in our LKr chamber test-experiment the real signal was 2.84 times tess than the predicted one. This resulted in a degradation of the experimental separation capability compared to the calculated result. We considered it as a result of the use of non-purified krypton. A better experimental separation capability (in agreement with Monte Carlo simulation) could be obtained with an optimized setup. The front-end electronics used in this test, for example, was designed for high capacitance detector 1131. References ItI 121 13 I 141
V.M. AuldKnkoct al.. Nucl. Instr~md Me&. A 316 (1992) 8. PA.Kutinich.JtNR~.El19497,~l994tRu~nedtoNIMl. l? Cantoni a al., Nucl. Insu. end Meth. A 344 ( 1994) 156. P. Gmtoni et al.. particle ldentilicaticn in a LKr ionizatiun chasnber by umlttpte induced cuftxm measurement3 u&g the shslx mmlydr of the signal. Pnx. 6th FairsMeeting on advanced detecmrs, La Biodote l&u d’ERm+ tmly. May 22 1994. ~S~U.F~.Ann.Rev.Nltcl.SEi.l3(1%3)1. 16) PA. Kutiuicb. R Toledo. JMR Rep. PI087-l 12. Dubna, 1987. 171 LecmyReaemcbtasmunenmtionCaietog. 1992. (81 G. Jaffe. Ann. Phyr. 42 t 1913) 303. 191 J. R. Gipsale. t&cl. IIISR. and Me&. I I I t 1973) 345. 1 I01 E Aprik et al.. Nuci. tnsfr, and MC&. A 327 t 1993125. I I I I PK. Lebedev. V.I. Pryaatchntkov,Nucl. Insw. msd Meth. A 327 t 19931 222. II21 D.Jemmee~J..Nuct. 1ma.awJMeth.A III tl973)2W. I 131 WM. Aulchwkoet at., Nuol. Insus and Me& A 289 t 19%) 468.