Properties of electromagnetic and hadronic showers measured with liquid argon calorimeters

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INSTRUMENIS

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160 ( 1 9 7 9 )

427-434

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NORIII-H()LLAND

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P R O P E R T I E S O F E L E C T R O M A G N E T I C AND H A D R O N I C S H O W E R S M E A S U R E D WFI'H L I Q U I D ARGON C A I , O R I M E T E R S A BABAEV ~

( LR ~, Geneva..S~,azerland W I) APt,L, J LNGI.LR, W ItOtMANN', K RAL.S(IINABLL" and 1) W F G t N L R "

Insnn~r li~ l-xpenmenwlle Kernph~stk ~k'r L'mvt'tsm, t ( l t t ) 14tl(l d('~ Kt'ltlfi)l${/llltlg32etllr141#15 Karlsru/lt'. Karls/141R'. (l~,rt#tall~ Re~.el',ed 25 August 1978 thgh energ) electrons, pions and protons ha',c been deter.ted ~lth t ~ o hqmd argon ~.alorinlcters Measurements ol the hnearn~ and energ.~ resolution ol the dcte~.tors ha',e been perlormed Mm.e one ol the dete~.tor,~ v, as ~,ubdn, ided into 80 sections, nleasuren~ents ol the lateral and longitudinal de,,elopmeni ol hadrorm. ~.a,,~ades were po'-,s~ble ['hc re',ulls are m good agreement v, Hh rVlonte ( a r m smlulattons The spatml and angular resolutum,, ol the dete~.tor ha,,e been e,,,tluated, t)plcal ',alues ,ire Io'/ .' = 17%, o~ = 20 m m and o"0 = 100 mrad at 20 ( J e \ ' tiadron energ~ t le~.trons and proton,, ~,ere idennfied b.~ exploiting their dfllerent s h o ~ e r development, the probabdit.~ ol nllsldcntll'it.dllt)n ',~d,, It,,,, than lO

1. Introduction Shower counters are of increasing ~mportance for particle detection m the field of high energ) phystcs. In the last )'ears, especially calorimeters using hqmd argon to sample the ionizatton produced by hadronic or electromagnetic cascades have turned out to be extremely useful devices This type of calortmeter, whose theory and design is well documented~-"), has proven to be a very versatile and accurate instrument for m e a s u n n g the energy of hadrons, electrons and photons. In this paper we describe the results of an experiment in which the shower development and shower fluctuations have been studied using segmented liquid argon calorimeters The energy-, poSZtlon- and angular resoluttons have been determined for hadron induced cascades. We further report on possibilities to distingutsh electrons and hadrons using the different longitudinal and lateral energy deposttton of the shower 2. Apparatus In th~s experiment, two shower counters called A and B were used. The calorimeters were opt> m~zed for detection o f electromagnetic and hadronic showers respecnvely

On leave ol absence Irom lnsulute Ior Theorehcal and Experimental Physics (ITt.P), Mo.~cov,, U S S R + Now at lm, titut fur Ph)'sik. I~niversltat Dortnlund, [)orb nlund, German)

The counter A consists ol 159 ~ron plates ol 1 5 m m thickness and 50× 50 c m ' area The 1.5 m m gap between the plates is filled with liquid argon. For full containment of electromagnenc showers, a total Iong~tudmal extension of the counter of about 15 radiation lengths was chosen The iron plates tire connected to ground and high voltage alternatlngly The charge signal was read out from the hv-plates wa a couphng capacitor To study the longitudinal development of a shower, the calormleter was subdtv~ded electromcally into 8 consecuttve sections of 10 up to 32 plates each Further details are given in table 1. The counter B has four identical modules o[ I 7 absorbt~on length depth each One module contains 41 ground plates and 40 hv-plates, both are made of 3 m m iron. bemg separated by 3 toni liquid argon gaps To obtain information on the lab eral energy dtstnbutton m hadromc showers, the hv-plates tire subdivtded into 10cm wide tron strtps m x and y-dtrecnon respectively, as shown in fig. 1 Besides the modular structure, no further Iongttudlnal subdivision was foreseen; consecutive horizontal and vemcal strips were connected electrically The calorimeter modules were contained m a cryostat of 2 6 x 1.3x I 3 n3 ~ mner d~menstons To fill the cryostat, the calortmeters were slowly cooled down to - 180°C using cold argon gas, the dewar was then filled wtth ~ 3 m ~ of hqutd argon During the m e a s u r e m e n t s , the regulation of the argon pressure was accomphshed by a refrtgerator

A

428

B A B A L V el al

r XHLI I P,iran~eters ol Ldlorlmeters A and B A

B 3 m m iron 324 103 × 103 cm 2 3 mm 192 61 68 8400 kg

Absorber plates

15 nlm iron

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The preamphfiers for the 88 electronic channels were mounted directly on the dewar. The amphtier design was smlllar to that used by Wdhs and Radeka:), wnh some minor modificauons The d~fferent chamber capacmes were matched to the amphfier input m~pedance with toro~dal fernte core transformers The preamplifier drove 20 m twisted pa~r lines connected to bipolar shaping amphfiers The amphfied s~gnals were d~gmzed by 10bn ADCs which were read out by an on-hne computer A test pulse apphed to the primary circuit allowed easy and stable cahbrauon of the whole chain. A schematic diagram of the electromcs chain for one channel is shown in fig. 2, further detads on the construction of the calorimeter modules and the read-out are gwen elsewhere 7) Measurements have been performed for two different confgurations at an external beam of the CERN PS In a first period, the calorimeter A was put m front of the hadron calorimeter B. Data taken in th~s period were used to derive properties of electromagneuc showers and to study poss~bdiues for particle ~denuficat~on wnh the shower counter

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In the tbllowmg period, the electron calortmeter was replaced by an mr-filled aluminum box, so particles entered the hadron calorimeter directly w~thout crossing any substantial amount of materlal. This configuration was used to investigate hadromc showers.

3. Detection of electromagnetic showers 3.1 PER!ORMANCE OF THE (ALORIMLTFR h g . 3 shows the response of the electron calorimeter A to tnc~dent electrons in the energy range from 3 to 13 GeV The high voltage apphed to the plates was 1.5 kV A good hneanty m the whole energy range was observed. The energy resolution after subtractton of amphfier noise is given by

(CrE/E)A = 9.5 °~,/x/'E

(E in GeV).

Th~s value has to be compared w~th to resoluuon measured for calorimeter B, whtch has twice the plate thickness and argon gap w~dth as A at 5 GeV, = 6.5~ ~noise subtracted corresponding to [(~E/E) = 14.5~/o/~,/E.

(eE/E)B

As ~s expected for showers (luctuatmg statistically, the energy resolution Js proportional to the square root of the samphng thickness at fixed electron energy.

3.2 SIIOWLR I)EYI.I,OPMENT In fig. 4, the longitudinal energy deposmon of an electromagnetic shower, as measured m the 8 sections of calorimeter A, ~s compared with pred~c-

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di~.t,oi] ol dndl)tic,il sho~er theor,es <~1 Ior the sho~er m,{x~I11UI11

tlons from a Monte C a r l o shower simulation p r o granl~ i~) ] h e energ} dependent longitudinal posttlon of shower center _: and the shower rnaxlmum 2m,, are plotted m fig 5. The stlower center is defined as 27 = V E,--, V L. where /',, ~s the energ$ deposned m section ~ of the calorlnleter, and ,:, ~s the center of section t To determine the shower m a x i m u m w~th reasonable accurracy, a parabola was fitted to the E,'s of the three secuons with m a x m m m energy deposition, the m a x m l u m of the parabola was taken as the shower maxm~um As shown m fig 5, the energy dependence o f z ...... is well described by z ..... = 1.04 ln(E.c) - : o , as predicted t ' r o m analyucal s h o w e r theories") e is the crmcal energy of the absorber material, v = 2 1 MoV, and :,, is a c o r r e c t i o n for additional material m front of the calorimeter, z,,--=-0 5 r I

4. Hadronic sho~er,, The properties of hadromc showers induced by plons and protons have been studied using the calorimeter B The energ) range covered by the experiment was 9 - 2 4 G e V lor protons and 4 - 1 9 G e V for plons

4.1. LI~I ,xRrh r h e total charge 0 deposited in the hadron calormleters ~s given by the sum 4-

20

0 = S2 - 1 1=1 Eq,,, where q,, is the charge measured in segment / of module i. l-lg 6 shows Q for incident protons m the energy range 9 - 2 4 G e V . No slgmficant dewalton,, from hnearlt) are observed 4.2 e / p R,',rlo Due to the different nature of hadromc and electromagneuc showers, the charge O measured ~n the calorimeter depends in general not only on the energy, but also on the type of the incoming pamcle The most prominent effect ~s a loss of charge for incident protons compared to electrons of the same kinetic energy, which is mostly due to the nuclear bindmg energy released by the hadronic shower and to leakage losses These effects have been discussed in detail by Gabriel and Schmldt 4) and by Fabjan et al") With the calorimeter B, the lbllowmg ratios tbr Q at equal kmeuc energy have been derived Q" = 117- 005 , Oe Q-7 Qp _ 1.6±0.10, __

o~ = 1.45+0.10. ~

These values are in good agreement with the resuits of other experiments and with Monte Carlo calculations 14 - I ")

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The subdivision of the calonmeter B into 4 modules w~th 20 transversal strips each allows to sample the long~tudmal shower development m steps of l 7 abs length and to measure the lateral shower distributions at 4 different depths. The resuit,', are compared wnh Monte Carlo shower s~mulauons using the high energy n u c l e o n - m e s o n transport code H E T C ~") Fig 7 shows the encrgy deposited m the central 10cm wide segment of the lour modules for mc~dent protons of 23 GeV The lateral energy d~stnbuuon ts plotted m fig 8 The energy, d e n s n y shown tn defined b~

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noise contribution, which was equl~,aleilt to 05 mm~mum ~omzmg pamcles

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In lig 9 the relative energy resolution o / / E ~s g~ven for mc~dent protons and pure,, The resolution can be paranletrtzed In the usual Iornl ¢v~ E = 75"'<,

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where tile 1 / x E term md~cates the effect ol statJMical fluctuations m tile shower development In fig. 10 the energy, resolutum is given ,is a funcuon of the absorber length : used At the lowest energy i l l GeV) an absorber depth of 1 5 abs. length is ,,ufl'iclent to contmn the ,,bower, while at the h~ghest energy (22 GeV) the resolunon improves w~th z even for ..->5 ab', lengths 4.5

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The strip structure ol the detector enables us to calculate the lateral posmon and the d~recuon of

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each md~vldual incoming part~cle from the energy d~stnbutJon m the cascade I'or th~s purpose, the lollowmg algorithm wa,, used, m a t]rst step, the x and y coordinate,, of the shower center m cach of thc four modules are den ~ed X-j =

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arctan0.<,

Fig. 11 shows the pos~t~on resolution a, =or as a function of the particles energy. The resolution a-~20 m m ~s small compared to the strip size of the calormleter ( l O 0 m m ) and improves slightly with energy A good parametnzatlon is given by a , [ m m ] = 3 1 ( I - 0 . 1 4 lnE

The resolution a, is roughly constant for particles hitting the central 8 cm of each strip, lot hLts ver~ close to the edges of the strips, the resolution worsens by 50% The angular resolution a , is given m lqg 12 A typical resolution ~s ao ~- 100 mrad ~- 6 ~ m agreement with the Monte Carlo results, which are meluded m fig. 12 a ~ ( E ) i s described by ao

.t-j = a, ~- Zj arctan 0~, is then fitted to the four points

~;~[~lt °

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[m]ad] = 210 ( I - 0 . 2 0 I n E [ G c V ] ) .

5. Particle identification From the pulse heights measured in each section of the shower counter, it ts possible to ~dentfl'y the shower type - electromagnetic or hadromc on the basis of statistical tests" ~ ,2) 10

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Fig 12 A n g u l a r r e s o l u u o n lot hadrom¢ sho~,ers T h e lull hne is predicted b.'¢ M o n t e Carlo c a h . u h m o n s 4)

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I ,g 13 Probability el, ol a proton to lulfil the ele~.tron selecnon ~.r,tena as a l u n c n o n ol the electron acceptam.e ~.~ Calorimeter A, 9 OeV electrum, resp protons

)>

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are calculated with well identified mcldent particles. in the followmg we study particle tdentlficatlon for fixed parttcle energy, in general, however, the energy depence of the (q,> and ~7,, has to be meluded ~ ) The test of the electron-hadron separation was perlornled with 9 GeV electrons and protons using calonmeter A tollowed by the fi)ur modules o1"

f

Let q = (q~, ., q,,,) be a vector characterizing a shower, e.g. the charges deposited in the ~ecuon 1 8 of counter A If the probability dmtribuuon p . (q) and p~ ,,,t(q) for hadromc and electromagnetic showers were known, one could calculate p . ( q ) ,

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counter B. A parncle was ~dentil]ed as an electron I["

PB~(q) > Pll(q)

and

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and ~ c e versa for protons To separate e g electrons from a proton background, the el'l'icienc,~ for electron r e c o g n m o n ts adju,,,ted with p ...... The separation power of the device ~,s then g~ven by the electron efficient) c, dn, qded b) the probabiht.~ e e for a proto11 to l'ullil the electron criteria i lg 13 shows c;, as a function of c using the 8 cllannels of counter A for separanon The separata}n power obtained is + 70 for 90% electron el'fic~ency and increases with decreasing & to ~ 500 Ibi g< --50% t~g 14 g~ves ~;p its a function of the detector length - used, tor fixed c = 5 0 % . Up to a length of ~ 10 rad lengths or I absorpnon length, the proton rejection increases eXl}onentially with 2, and then levels off A lurther mlpro~ement ~ posslb]e

b)

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Monte Carlo calculations shows that the currently available cascade s m m l a n o n models are m reasonable a g r e e m e n t with the data The calorimeters allow to measure the energy', ~mpact p o s m a n and d~recnon of a hadron wnh an iiccurac) of o'~/E = 17%, o, ---=20 m m and o-#----1(}0 mrad at 2 0 G e V parncle energy With increasing energy, the energy resolunon mlproves according to E ', ~hlle the spatial and arigular resolunon exhibit a I n { l i E ) dependence We acknowledge the hell) of l}rs. t M o n n i g and tl Schneider during data taking We would I~ke to thank II. Plucker, ti Ke~m and V Wesche for construction and running ot" the cr2,ogenlcs and B Friend for h~s help tn setting up the electromcs This work was supported b,,< the Bundesnlinisterrain fur t orschung und Technologte, G e r m a n s

shower

de,,eh}pnlent In fig 15 c,, is plotted its
licicnc) of 80% 6. ( onchl~,iOll,~ T ~ o liquid argon calorimeters ~ere used to dclet'l cicclrt}llS, plons dnd protons lil the cncrg) lallgC 3-]3 (Je\ , 4-19 Go\ iilld 9-24 {Je\ respectl~cl) I o r ,ill particles, the charge signal depend,> Iiiledr[) t)ll t h e kinetic etlelg) o[ the illcldellI p4rtitle lhc signal ho~e~cl depends Oll the particle t.~ pc 0<;Q-:Qn

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