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The liquid krypton calorimeter of NA48: First operation results
Nuclear Instruments and Methods in Physics Research A 409 (1998) 570—574
The liquid krypton calorimeter of NA48: first operation results F. Costantini Dipartimento di Fisica, Universita% di Pisa, and I.N.F.N. Sezione di Pisa, I-56100 Pisa, Italy
Representing the NA48 Collaboration1
Abstract The first technical run of the complete NA48 experimental apparatus took place in 1996. The first operation results of the full size liquid Krypton electromagnetic calorimeter as energy resolution and p0 mass resolution are presented in this paper. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction The NA48 experiment is designed [1] to improve the measurement of the direct CP-violation observable double ratio R:
read-out FADC’s tags the protons hitting the K target to allow the distinction between K vs. S S K decays [2]. L
K0Pp0p0 K0Pp`p~ L R" L . (1) K0Pp0p0 K0Pp`p~ S S A 10~3 deviation from unity in the double ratio R determines +2]10~4 variation on the known parameter Re(e@/e) through the relation
2. Physics requirements on the calorimeter
Re(e@/e)"1 (1!R). (2) 6 To achieve a 10~3 precision on R, about 106K0P L p0p0 decays need to be collected. Two simultaneous and almost colinear beams are used, one for K and L one for K , so that systematic errors are reduced S measuring concurrently the four decay modes. A scintillator hodoscope equipped with 1 GHz
f p(E)/E+1% at 10 GeV and a constant term )0.5%, f a position resolution p +p +1 mm, x y f a capability to operate at an instantaneous single particle flux of about 1 MHz.
N
1 Cagliari, Cambridge, CERN, Dubna, Edinburgh, Ferrara, Firenze, Mainz, Orsay, Perugia, Pisa, Saclay, Siegen, Torino, Warszawa, Wien.
Neutral K decays are measured in the electromagnetic calorimeter only. The main requirements set on the calorimeter by the aim of reconstructing K0Pp0p0P 4 photon decays are
A calorimeter time resolution better than 0.3 ns provides, in conjunction with the tagger time information, the way to tell the K0 decays from the S K0 ones. The photon energy of interest ranges L between 3 and 100 GeV.
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 1 6 8 - 9 0 0 2 ( 9 7 ) 0 1 3 2 2 - 3
F. Costantini/Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 570—574
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2.1. Energy and position resolutions A transverse scale accuracy of 0.1 mm over the 1.3 m radius of the calorimeter is needed to control the systematic error arising from the energy scale difference between the p0p0 decays measured in the electromagnetic calorimeter and the p`p~ decays measured in the spectrometer. The K0Pp0p0 decay vertex distance from the calorimeter, is given by the following expression where the 4 shower energies (E ) and positions i (x , x ) are measured by the calorimeter: i j 1 Z " J + E E ((x !x )2#(y !y )2). (3) VTX M i j i j i j K iyj The shower energy and position resolutions and their uniformity all over the whole calorimeter determine the error on Z . In fact, a systematic VTX energy scale error on *Z /Z of +2]10~4 VTX VTX induces an error on *R/R of +6]10~4 [3], if a fiducial region on Z of 2.5K mean free paths VTX S is chosen. A good energy resolution minimizes also the K0P3p0 background contribution (when only 4 photons are detected) to the K P2p0 signal. L This is the third most important source of error. 2.2. The time resolution The precise measurement of the 4 shower arrival times in conjunction with the time measurement from the tagger is the only way to tell a K neutral S decay from a K one. A good calorimeter time L resolution is therefore essential for the experiment. The tagger detects with a precision better than 250 ps the arrival of protons on the K production S target, therefore a similar time resolution is required from the calorimeter. An independent time measurement is performed by an auxiliary detector made by a plane of bundles of scintillating fibers. This detector is located inside the calorimeter at a depth of about eight radiation lengths [4].
3. The calorimeter structure The cross section of the calorimeter structure is an octagon with an inscribed circle of 128 cm
Fig. 1. Sketch of the cell geometry and of how a spacer plate gives a 48 mrad angle to ribbons.
radius. The active length of the liquid krypton is 120 cm, which corresponds to about 26X . About 0 13 500 cells of 2]2 cm2 cross section are obtained stretching 1.8 cm wide ribbons made of Cu—Be alloy between two precisely machined stesalit front and back plates. A cell is delimited by two grounded ribbons 2 cm apart and has an anode central ribbon (Fig. 1). As the Molie`re radius of the liquid krypton is 4.7 cm, up to about 40% of the shower total energy can be deposited in one cell (Fig. 2). Five stesalit plates spaced every 20.5 cm along the ribbon length act as ribbon spacers. The ribbons are kept in position by slots precisely machined on the plates to control the distance between ribbons to better than 50 lm. The spacer plates are transversally displaced one with respect to the other producing a $48 mrad longitudinal zig-zag to the ribbons. This accordeon shape reduces to about $0.5% a small drop in response uniformity across the cell, when the shower axis hits the central anode ribbon.
VII. TRENDS IN CALORIMETRY
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F. Costantini/Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 570—574
their minimum noise figure at the liquid krypton temperature of 120 K. The energy-equivalent noise is 4 MeV when the FADC input shaping time is 120 ns and the HV is 5 kV. The precision and the stability of the calibration system allows to intercalibrate the response of the 13 500 channels to better than 0.5%. 3.2. The preamplifier HV protection system
Fig. 2. Lego plot of shower energy deposition in the calorimeter cells. The two X and ½ projections are used on-line for a 40 MHz pipe-line trigger.
The longitudinal ribbon structure has been designed to allow a tower read-out of the calorimeter, in order to resolve shower overlapping and to reduce the effects of accidentals. The tower read-out is fully exploited off-line, while on-line the pipelined trigger on the neutral K decays is based on shower counting performed on the X and ½ projections of the energy depositions (Fig. 2) [5]. A projective geometry pointing to about 110 m upstream the calorimeter, approximately halfway the K decay S fiducial region, is obtained increasing linearly the cell transverse size (both in X and ½), starting from the front plate and adding up to #1.1% at the back plate. In this way the shower response uniformity, both on the radius of the impact point and on the depth of the shower maximum, is improved. This is essential in order to achieve the quoted accuracy in the energy scale. An exhaustive description of the calorimeter structure is given in Ref. [6].
A protection system is installed to prevent preamplifiers from being damaged by possible HV discharges between ribbons during the initial HV ramp up and conditioning. The system grounds simultaneously all preamplifier inputs. Tests have shown that continuous HV discharges of the 3 nF decoupling capacitors up to 7 KV “simulated” in the ribbon structure leave preamplifiers unaffected, when the protection system is activated. The system, which was not implemented in the calorimeter prototype, proved to be effective and reliable during the first year of operation.
4. First operation results During the first data taken in 1996 only a small central region of 384 cells of the calorimeter was equipped with the final single-cell read-out FADC system [8]. All the rest of the calorimeter cells had a coarser granularity read-out grouping 16 cells (2X]8½) into one FADC channel. This choice was dictated by the available number of FADCs. In this way it was possible to measure for the first time both the energy deposition in the whole calorimeter with a kaon beam and the energy and time resolutions of the full size calorimeter exposed to an electron beam inpinging in the central region equipped with the single cell read-out.
3.1. The front end electronics
4.1. The energy resolution
The preamplifiers and the calibration system electronics [7] are located on the back plane of the calorimeter, plugged on the pins of the ribbon back ends, and immersed in the liquid krypton. The preamplifiers are based on Si JFET’s which have
The energy resolution measured with 50 GeV electron beam is p(E)/E"0.69% and matches the value measured with the prototype calorimeter [9] of p(E)/E"J(a2/E#b/E2#c2) with a"3.5%, b"40 MeV and c"0.42% as shown in Fig. 3.
F. Costantini/Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 570—574
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about 1 MeV expected with the final single-cell read-out. 4.2. The time resolution A time resolution p(t)"263 ps has been measured for 50 GeV calibration electrons inpinging on the calorimeter central region equipped with single cell FADC read-out. A paper focussed on the time resolution of the NA48 calorimeter has been presented at this Conference and the reader is referred to it for further details [10]. 4.3. The position resolution
Fig. 3. Energy resolution of the full-size calorimeter region equipped with single-cell read-out and comparison with the prototype one.
The position resolution is obtained by comparing the position of the center of gravity (COG) of the shower in the calorimeter with the electron impact point as reconstructed by the spectrometer. The position resolution measured by the full size calorimeter matches that one measured by the prototype [9] as function of the beam energy, parametrized as p(x)+p(y)+(4.2/JE(GeV)=0.6) mm.
5. Summary A quasi-homogeneous electromagnetic liquid krypton calorimeter with tower read-out has been built by the NA48 Collaboration at CERN. The sensitive volume of more than 6 m3 is segmented in about 13 500 longitudinal cells. The first operation results presented show that prototype energy, position and timing resolutions are well matched by the full-size calorimeter.
Acknowledgements Fig. 4. p0 mass resolution measured grouping 2X]8½ cells of the calorimeter.
A p0 mass resolution with p(m)"3.0 MeV was obtained (see Fig. 4) measuring the photon energies with the above-mentioned read-out of 16 cell grouping over the entire calorimeter. This resolution is in agreement with the p0 mass resolution of
I would like to thank all the collaborators and the technical staff of the NA48 Collaboration, in particular those who contributed to the construction of the calorimeter. A special thank to the Technical Coordinator Dr. D. Schinzel who succeeded in the remarkable coordination of a very tight schedule construction. The Elba Conference
VII. TRENDS IN CALORIMETRY
574
F. Costantini/Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 570—574
this year has been very interesting and stimulating, extremely well organized and always efficiently and smoothly manned in a magnificent environment. It is a real pleasure to thank the whole Organizing Committee and in particular Prof. Angelo Scribano, Prof. G. Batignani, Mrs. C. Corazza and Mrs. L. Lilli for making all this possible. References [1] G.D. Barr et al., Proposal for a precision measurement of e@/e in CP violating K0P2p decays, CERN/SPSC/9022/P253. [2] T. Beier, Nucl. Instr. and Meth. A 360 (1995) 390. [3] I. Mannelli, NA48 Collaboration Meeting, Gmunden, 1992.
[4] NA48 Internal Notes 94-18, 97-22. [5] I. Mikulec, these Proceedings (7th Pisa Meeting on Adanced Detectors, La Biodola, Isola d’Elba, Italy, 1997) Nucl. Instr. and Meth. A 409 (1998) 662. [6] S. Palestini, An electromagnetic calorimeter based on liquid Krypton with tower read-out structure, presented at the 1966 IEEE Nuclear Science Symposium, November 3—9, 1996, Anaheim, CA, and also NA48 96-24 Internal Note. [7] C. Cerri, The NA48 liquid Krypton calorimeter: electrode structure, front-end electronics and calibration, Proceedings of the VI International Conference on Calorimetry in H.E.P. Frascati, June 1996, pp. 841—848. [8] B. Hallgren et al., IEEE Trans. Nucl. Sci. 43 (1996) 1605. [9] G.D. Barr et al., Nucl. Instr. and Meth. A 370 (1996) 413. [10] S. Cre´pe´, in: [5], p. 575.
The liquid krypton calorimeter of NA48: first operation results F. Costantini Dipartimento di Fisica, Universita% di Pisa, and I.N.F.N. Sezione di Pisa, I-56100 Pisa, Italy
Representing the NA48 Collaboration1
Abstract The first technical run of the complete NA48 experimental apparatus took place in 1996. The first operation results of the full size liquid Krypton electromagnetic calorimeter as energy resolution and p0 mass resolution are presented in this paper. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction The NA48 experiment is designed [1] to improve the measurement of the direct CP-violation observable double ratio R:
read-out FADC’s tags the protons hitting the K target to allow the distinction between K vs. S S K decays [2]. L
K0Pp0p0 K0Pp`p~ L R" L . (1) K0Pp0p0 K0Pp`p~ S S A 10~3 deviation from unity in the double ratio R determines +2]10~4 variation on the known parameter Re(e@/e) through the relation
2. Physics requirements on the calorimeter
Re(e@/e)"1 (1!R). (2) 6 To achieve a 10~3 precision on R, about 106K0P L p0p0 decays need to be collected. Two simultaneous and almost colinear beams are used, one for K and L one for K , so that systematic errors are reduced S measuring concurrently the four decay modes. A scintillator hodoscope equipped with 1 GHz
f p(E)/E+1% at 10 GeV and a constant term )0.5%, f a position resolution p +p +1 mm, x y f a capability to operate at an instantaneous single particle flux of about 1 MHz.
N
1 Cagliari, Cambridge, CERN, Dubna, Edinburgh, Ferrara, Firenze, Mainz, Orsay, Perugia, Pisa, Saclay, Siegen, Torino, Warszawa, Wien.
Neutral K decays are measured in the electromagnetic calorimeter only. The main requirements set on the calorimeter by the aim of reconstructing K0Pp0p0P 4 photon decays are
A calorimeter time resolution better than 0.3 ns provides, in conjunction with the tagger time information, the way to tell the K0 decays from the S K0 ones. The photon energy of interest ranges L between 3 and 100 GeV.
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 1 6 8 - 9 0 0 2 ( 9 7 ) 0 1 3 2 2 - 3
F. Costantini/Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 570—574
571
2.1. Energy and position resolutions A transverse scale accuracy of 0.1 mm over the 1.3 m radius of the calorimeter is needed to control the systematic error arising from the energy scale difference between the p0p0 decays measured in the electromagnetic calorimeter and the p`p~ decays measured in the spectrometer. The K0Pp0p0 decay vertex distance from the calorimeter, is given by the following expression where the 4 shower energies (E ) and positions i (x , x ) are measured by the calorimeter: i j 1 Z " J + E E ((x !x )2#(y !y )2). (3) VTX M i j i j i j K iyj The shower energy and position resolutions and their uniformity all over the whole calorimeter determine the error on Z . In fact, a systematic VTX energy scale error on *Z /Z of +2]10~4 VTX VTX induces an error on *R/R of +6]10~4 [3], if a fiducial region on Z of 2.5K mean free paths VTX S is chosen. A good energy resolution minimizes also the K0P3p0 background contribution (when only 4 photons are detected) to the K P2p0 signal. L This is the third most important source of error. 2.2. The time resolution The precise measurement of the 4 shower arrival times in conjunction with the time measurement from the tagger is the only way to tell a K neutral S decay from a K one. A good calorimeter time L resolution is therefore essential for the experiment. The tagger detects with a precision better than 250 ps the arrival of protons on the K production S target, therefore a similar time resolution is required from the calorimeter. An independent time measurement is performed by an auxiliary detector made by a plane of bundles of scintillating fibers. This detector is located inside the calorimeter at a depth of about eight radiation lengths [4].
3. The calorimeter structure The cross section of the calorimeter structure is an octagon with an inscribed circle of 128 cm
Fig. 1. Sketch of the cell geometry and of how a spacer plate gives a 48 mrad angle to ribbons.
radius. The active length of the liquid krypton is 120 cm, which corresponds to about 26X . About 0 13 500 cells of 2]2 cm2 cross section are obtained stretching 1.8 cm wide ribbons made of Cu—Be alloy between two precisely machined stesalit front and back plates. A cell is delimited by two grounded ribbons 2 cm apart and has an anode central ribbon (Fig. 1). As the Molie`re radius of the liquid krypton is 4.7 cm, up to about 40% of the shower total energy can be deposited in one cell (Fig. 2). Five stesalit plates spaced every 20.5 cm along the ribbon length act as ribbon spacers. The ribbons are kept in position by slots precisely machined on the plates to control the distance between ribbons to better than 50 lm. The spacer plates are transversally displaced one with respect to the other producing a $48 mrad longitudinal zig-zag to the ribbons. This accordeon shape reduces to about $0.5% a small drop in response uniformity across the cell, when the shower axis hits the central anode ribbon.
VII. TRENDS IN CALORIMETRY
572
F. Costantini/Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 570—574
their minimum noise figure at the liquid krypton temperature of 120 K. The energy-equivalent noise is 4 MeV when the FADC input shaping time is 120 ns and the HV is 5 kV. The precision and the stability of the calibration system allows to intercalibrate the response of the 13 500 channels to better than 0.5%. 3.2. The preamplifier HV protection system
Fig. 2. Lego plot of shower energy deposition in the calorimeter cells. The two X and ½ projections are used on-line for a 40 MHz pipe-line trigger.
The longitudinal ribbon structure has been designed to allow a tower read-out of the calorimeter, in order to resolve shower overlapping and to reduce the effects of accidentals. The tower read-out is fully exploited off-line, while on-line the pipelined trigger on the neutral K decays is based on shower counting performed on the X and ½ projections of the energy depositions (Fig. 2) [5]. A projective geometry pointing to about 110 m upstream the calorimeter, approximately halfway the K decay S fiducial region, is obtained increasing linearly the cell transverse size (both in X and ½), starting from the front plate and adding up to #1.1% at the back plate. In this way the shower response uniformity, both on the radius of the impact point and on the depth of the shower maximum, is improved. This is essential in order to achieve the quoted accuracy in the energy scale. An exhaustive description of the calorimeter structure is given in Ref. [6].
A protection system is installed to prevent preamplifiers from being damaged by possible HV discharges between ribbons during the initial HV ramp up and conditioning. The system grounds simultaneously all preamplifier inputs. Tests have shown that continuous HV discharges of the 3 nF decoupling capacitors up to 7 KV “simulated” in the ribbon structure leave preamplifiers unaffected, when the protection system is activated. The system, which was not implemented in the calorimeter prototype, proved to be effective and reliable during the first year of operation.
4. First operation results During the first data taken in 1996 only a small central region of 384 cells of the calorimeter was equipped with the final single-cell read-out FADC system [8]. All the rest of the calorimeter cells had a coarser granularity read-out grouping 16 cells (2X]8½) into one FADC channel. This choice was dictated by the available number of FADCs. In this way it was possible to measure for the first time both the energy deposition in the whole calorimeter with a kaon beam and the energy and time resolutions of the full size calorimeter exposed to an electron beam inpinging in the central region equipped with the single cell read-out.
3.1. The front end electronics
4.1. The energy resolution
The preamplifiers and the calibration system electronics [7] are located on the back plane of the calorimeter, plugged on the pins of the ribbon back ends, and immersed in the liquid krypton. The preamplifiers are based on Si JFET’s which have
The energy resolution measured with 50 GeV electron beam is p(E)/E"0.69% and matches the value measured with the prototype calorimeter [9] of p(E)/E"J(a2/E#b/E2#c2) with a"3.5%, b"40 MeV and c"0.42% as shown in Fig. 3.
F. Costantini/Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 570—574
573
about 1 MeV expected with the final single-cell read-out. 4.2. The time resolution A time resolution p(t)"263 ps has been measured for 50 GeV calibration electrons inpinging on the calorimeter central region equipped with single cell FADC read-out. A paper focussed on the time resolution of the NA48 calorimeter has been presented at this Conference and the reader is referred to it for further details [10]. 4.3. The position resolution
Fig. 3. Energy resolution of the full-size calorimeter region equipped with single-cell read-out and comparison with the prototype one.
The position resolution is obtained by comparing the position of the center of gravity (COG) of the shower in the calorimeter with the electron impact point as reconstructed by the spectrometer. The position resolution measured by the full size calorimeter matches that one measured by the prototype [9] as function of the beam energy, parametrized as p(x)+p(y)+(4.2/JE(GeV)=0.6) mm.
5. Summary A quasi-homogeneous electromagnetic liquid krypton calorimeter with tower read-out has been built by the NA48 Collaboration at CERN. The sensitive volume of more than 6 m3 is segmented in about 13 500 longitudinal cells. The first operation results presented show that prototype energy, position and timing resolutions are well matched by the full-size calorimeter.
Acknowledgements Fig. 4. p0 mass resolution measured grouping 2X]8½ cells of the calorimeter.
A p0 mass resolution with p(m)"3.0 MeV was obtained (see Fig. 4) measuring the photon energies with the above-mentioned read-out of 16 cell grouping over the entire calorimeter. This resolution is in agreement with the p0 mass resolution of
I would like to thank all the collaborators and the technical staff of the NA48 Collaboration, in particular those who contributed to the construction of the calorimeter. A special thank to the Technical Coordinator Dr. D. Schinzel who succeeded in the remarkable coordination of a very tight schedule construction. The Elba Conference
VII. TRENDS IN CALORIMETRY
574
F. Costantini/Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 570—574
this year has been very interesting and stimulating, extremely well organized and always efficiently and smoothly manned in a magnificent environment. It is a real pleasure to thank the whole Organizing Committee and in particular Prof. Angelo Scribano, Prof. G. Batignani, Mrs. C. Corazza and Mrs. L. Lilli for making all this possible. References [1] G.D. Barr et al., Proposal for a precision measurement of e@/e in CP violating K0P2p decays, CERN/SPSC/9022/P253. [2] T. Beier, Nucl. Instr. and Meth. A 360 (1995) 390. [3] I. Mannelli, NA48 Collaboration Meeting, Gmunden, 1992.
[4] NA48 Internal Notes 94-18, 97-22. [5] I. Mikulec, these Proceedings (7th Pisa Meeting on Adanced Detectors, La Biodola, Isola d’Elba, Italy, 1997) Nucl. Instr. and Meth. A 409 (1998) 662. [6] S. Palestini, An electromagnetic calorimeter based on liquid Krypton with tower read-out structure, presented at the 1966 IEEE Nuclear Science Symposium, November 3—9, 1996, Anaheim, CA, and also NA48 96-24 Internal Note. [7] C. Cerri, The NA48 liquid Krypton calorimeter: electrode structure, front-end electronics and calibration, Proceedings of the VI International Conference on Calorimetry in H.E.P. Frascati, June 1996, pp. 841—848. [8] B. Hallgren et al., IEEE Trans. Nucl. Sci. 43 (1996) 1605. [9] G.D. Barr et al., Nucl. Instr. and Meth. A 370 (1996) 413. [10] S. Cre´pe´, in: [5], p. 575.