A calorimeter with wavelength shifter read-out for particles at small angles in proton-antiproton collisions

Nuclear Instruments and Methods 200 (1982) 195-198 North-Holland Publishing Company

195

A CALORIMETER WITH WAVELENGTH SMALL ANGLES

SHIFTER READ-OUT IN PROTON-ANTIPROTON COLLISIONS

FOR PARTICLES

AT

C. B A C C I , R. B E R N A B E I , F. C E R A D I N I , S. D ' A N G E L O , F. L A C A V A , M. M O R I C C A , L. P A O L U Z I a n d G. S A L V I N I Institute of Physics of the University of Rome and lstituto Nazionale di Fisica Nucleate (INFN). Sezione Roma, Rome, Italy Received 18 January 1982

We report the results of a test of two total absorption calorimeters with wavelength shifter read-out. They are now installed in the very forward parts of the UAI experiment at the proton-antiproton collider at CERN. The response and resolution obtained with electrons and hadrons in the energy range 2-10 GeV are presented.

I. Introduction

2. Details of the design

We describe in this paper the structure and the results of the calorimeters which have been built to measure the energy of particles emitted at very small angles in the interactions produced in the 540 GeV p r o t o n - a n t i p r o t o n collider at C E R N . These are the so called very forward calorimeters of the UA1 experiment [1,2]. The layout of one arm of the very forward calorimeter is shown in fig. 1. The two forward calorimeters are now operating in the experiment.

Each calorimeter is divided into two parts. The first one, indicated as "electromagnetic calorimeter", is built with layers of lead and scintillator; the second, the "hadronic calorimeter", with layers of iron and scintillator. The choice of materials like lead and iron is convenient in order to distinguish electrons and photons, which develop their showers in lead, from hadrons. The electromagnetic calorimeter is divided into four sectors in q, with 90 ° opening angles around the beam pipe, and four modules along the beam direction. Each module is a sandwich of sheets of lead, 3 mm thick, and

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Fig. I. Layout of the very forward part of the UAI experiment at the proton-antiproton collider at CERN. The same layout is on the other side of the apparatus. 0167-5087/82/0000-0000/$02.75

© 1982 North-Holland

196

C. Bacci et aL / Culortmeter with wacelength

Table 1 Distribution of absorbers and scintillators in the four modules. ~M. 2cmAl+3mm Pb+8X(6mmSc+3mmPb) -,- I cm Fe I cm F e + 3 mm P b + 9 x ( 6 mm Sc+3 mm Pb) + 6 mm Sc 1 cm F e + 3 mm P b + 9 X ( 6 mm Sc+3 mm Pb) + 6 mm S c + l c m Fe 1 cm F e + 3 mm Pb + 8 x ( 6 mm Sc + 3 mm Pb) +3 m m P b + 2 c m Fe

I

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5

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mm S c + 4 c m F e ) X 4 + 10 mm Sc mm S c + 4 c m F e ) X 4 + 1 0 mm Sc mm S c + 4 c m F e ) X 4 + 10 mm Sc mm Sc+4 cm F e ) X 4 + 10 mm Sc mm S c + 4 c m F e ) X 4 + 10 mm Sc

scintillators, 6 m m Plexipop 1922, clamped between external iron a n d a l u m i n i u m plates. The total thickness of the four modules is equivalent to 24 radiation lengths. In table 1 we give a distribution of the absorbers a n d scintillators in the four modules. T h e scintillation light is collected from one side, t h r o u g h a light wavelength shifter [31, this being BBQ

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d o p e d plexiglass (60 m g / l ) and a twisted light guide, s h o w n in fig. 2c. Each module is viewed by one Philips XP2008 photomultiplier. The anode of the photomultipliers is connected to a two-stage charge amplifier [4] with amplifications x 25 a n d x 6 5 0 respectively, ensuring a linear response over a large d y n a m i c range. Low amplitude pulses are read on the X 6 5 0 o u t p u t while large pulses saturate the x 6 5 0 o u t p u t and are read on the × 2 5 output. The amplifier outputs are sent through 80 m long twisted pair cables individually shielded to a differential receiver [5] whose o u t p u t is then read out with a 12-bit charge sensitive A D C LRS 2282. T h e receiver foresees also linear c o m b i n a t i o n s between different channels which will be used for triggering purposes, e.g. total energy deposited in the modules of one given sector or sum over all sectors. The h a d r o n i c calorimeter is also divided into four sectors with ~ = 90 ° opening angles. It has five modules along the b e a m direction. Each module is an alternation of 4 c m iron a n d l cm Piexipop 1922 scintillator as indicated in table 1 with a total length of 100 cm. The read-out is also in this case, as for the electromagnetic calorimeter, made from one side through a light wavelength shifter. Each module is viewed by one EMI 98 i I A photomultiplier. There is no amplifier connected to the a n o d e and the read-out chain is the same as for the electromagnetic calorimeter. T h e long-term stability of each component, photomultiplier and amplifier gain, A D C conversion factor, should be continuously monitored to guarantee the con-

....

b~ Fig. 2. Detailed design of one of the two calorimeters under test. (a) General view; (b) front view; (c) twisted light guide for light collection.

C Bacci et al. / Calorimeter with wavelength shifter read-out stancy of the ratio of deposited energy to A D C channel number for each module of the calorimeter. For this purpose a pulsed uv laser [6] is used a s a light reference source. The laser light is sent to each module through an optical fiber. The stability of the laser light itself is monitored by comparison with the light produced by an 241Am source in a NaI crystal. Since the calorimeters are divided only into four sectors, they cannot have a significant space resolution. To have information on the position and multiplicity of the incident particles we use two thin drift chamber pairs with image read-out. One pair is placed between the first and second module of each electromagnetic calorimeter; the other between the first and second module of each iron calorimeter (see figs. 1,2). The detailed design and performances ' o f these chambers were reported in ref. 7.

3. Tests of the calorimeters

Both calorimeters have been exposed to a test beam of the C E R N PS containing electrons, hadrons and muons of momenta from 2 to l0 G e V / c . The beam has a typical A p / p value of 1%, a #/~r ratio from 2% to 5% and an e/~r ratio of the order of l0 -2. The separation between electrons and hadrons was performed by means of two gas Cherenkov counters. Muon definition was obtained with counters placed behind a I m iron wall. The beam incident particle position was defined by a 2 × 2 cm 2 counter. We have measured the attenuation of the light through the Plexipop by measuring the response with the particles incident in different points of the calorimeters. The attenuation length resulted to be ~. = ( 154 -+- 20) cm, as shown in fig. 3. Data were taken with electrons of 2, 5, 7,5, l0 GeV incident in the center of each sector. Fig. 4 shows the relative energy resolution as a function of the beam

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Fig. 4. Relative resolution as a function of the energy for electrons: (a) calorimeter now installed on antiproton arm; (b) calorimeter now installed on proton arm.

energy for all the different sector. As we can see they follow with good approximation the same behaviour AE/E= ~ / ~ + C, being C a constant < 1 0 - : and K spanning a region from 0.18 to 0.22. By A E we mean the standard deviation of the energy distribution. The same energies were used with a beam of pions incident in the center of each sector. Fig. 5 summarizes the energy resolution of the calorimeters for hadrons. The hadron shower will develop both in the lead and in the iron calorimeter. The overall resolution we have found is in the range A E / E = (0.60-0.76)/v/-~GeV) . The total deposited energy, E, was estimated by the expression 4

5

E = G ] E g, Ai+G2 ~ fjAj, t=l

(1)

j=l

where At = (ADC-pedestal) channel, G], G 2 = total energy lost by a muon in the lead and iron calorimeters respectively, g,,fj = number of muons (g,, f~ "~ 1) corresponding to a single A D C channel. HAOI+xj

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Fig. 5. Percentage resolution as a function of the energy for hadrons (a) calorimeter installed on antiproton arm; (b) calorimeter installed on proton arm.

198

C. Bacci et aL / Calorimeter with wavelength shifter read-out

?E,n(GeV)• ELM 10~- " H A

Where the first sum refers to the lead modules, a n d the second sum to the iron ones. With the data shown in fig. 6 we can verify the linearity between the incident energy of the b e a m and the reconstructed energy. In this case the b e a m was incident orthogonally on the center of each sector. Finally to study the response as a function of the position of the incident particle we have taken data with electrons a n d h a d r o n s incident in different positions. F r o m the results shown in fig. 7, one can see that the response is quite u n i f o r m also when the shower develops in two different sectors.

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Fig. 6. Mean values of the reconstructed energy as a function of the beam energy for a single sector hit in the center.

T h e values G~g, a n d G2f: have been o b t a i n e d by requiring the squared sum of errors to be a m i n i m u m for all b e a m energies, E b.

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W e acknowledge the dedicated a n d c o m p e t e n t help of our technical staff in R o m e and of the PS a n d SPS staff in C E R N . We t h a n k C. R u b b i a for suggestions, and all our colleagues of the U A I Collaboration for advice a n d help during design a n d test.

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References

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W e have presented the test results of two calorimeters designed to detect the forward jets in the C E R N UA1 experiment. T h e results o b t a i n e d and reported in this note satisfy the requirement of this experiment. More in general, these calorimeters are in our opinion of interest when it is necessary to measure a n d to distinguish the energy deposited by electronic a n d h a d r o n i c showers. As is well known, this is a problem of increasing i m p o r t a n c e in experiments with high energy colliders.

~in % ' ]

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4. Conclusions

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Fig. 7. Map of the percentage difference between reconstructed and incident energy: for electrons (lower value) and hadrons (upper value). Data refer to the calorimeter in the antiproton arm.

[1] A. Astbury et al., CERN/SPSC/78-06, SPC/P92(1978). [2] C. Bacci, R. Bernabei, F. Ceradini, S. d'Angelo. F. Lacava, M. Moricca, L. Paoluzi and G. Salvini, re-print n.784, Inst. of Phys., Rome University (1981). [3] G. Keil, Nucl. Instr. and Meth. 83 (1970) 145. [4] J. Colas and J.C. Lacotte, Nucl. Instr. and Meth. 176 (1980) 283. [5[ B. Aubert et al., Nucl. Instr. and Meth. 176 (1980) 195. [6] The laser system has been particularly studied by the Centre d'Etudes Nucl6aire, Saclay, France. [7] C. Bacci, R. Bernabei, F. Ceradini, S. d'Angelo, F. Lacava, M. Moricca, L. Paoluzi and G. Salvini, Phys. Scripta 23 (1981) 662.