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A RICH detector for the NA62 very rare kaon decay experiment at CERN
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 595 (2008) 47–50
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A RICH detector for the NA62 very rare kaon decay experiment at CERN F. Bucci ` di Firenze e Sezione dell’INFN di Firenze, via G. Sansone 1, I-50019 Sesto F.(FI), Italy Universita
a r t i c l e in fo
abstract
Available online 20 July 2008
The NA62 experiment is designed to measure the very rare kaon decay Kþ ! pþ nn at the CERN SPS with a statistical precision of less than 10%. The Standard Model prediction for the branching ratio is ð8 1Þ 1011 . One of the challenging aspects of the experiment is the suppression of the Kþ ! mþ nm background at the 1012 level. To satisfy this requirement we need a Ring Imaging Cherenkov Detector able to separate p from m in the momentum range between 15 and 35 GeV=c with a muon suppression factor of at least 102 . We discuss the design and the construction of a RICH detector with a very long focal length ð17 mÞ, neon radiator gas at atmospheric pressure and 2000 fast photodetectors. With a total time resolution of the order of 100 ps this RICH detector can also be used to measure the arrival time of the pion and provide the experiment’s trigger. & 2008 Elsevier B.V. All rights reserved.
Keywords: RICH PID
1. The NA62 experiment The NA62 experiment is designed to measure the Kþ ! pþ nn branching fraction at the CERN Super Proton Synchrotron (SPS). The Standard Model prediction for the branching ratio is ð8 1Þ 1011 . The experiment aims to collect in two years of data taking about 80 Kþ ! pþ nn events with a signal to background ratio S=B ’ 10. We propose to employ a 75 GeV=c momentum beam of positive kaons to be derived from a flux of 400 GeV=c protons. The NA62 detector (Fig. 1) consists of:
A detailed description of the NA62 layout can be found in Refs. [1,2].
2. RICH The RICH detector must fulfill the following requirements:
separate p from m in the momentum range from 15 to
a differential Cherenkov counter (CEDAR) placed in the incoming beam to tag kaons;
thin silicon micro-pixel detectors (GIGATRACKER) able to work at 1 GHz to measure the momentum of the incoming particle;
a set of ring anticounters (CHANTI) to veto charged particles coming from the collimator;
35 GeV=c with a muon suppression factor of at least 102 to help suppressing the background coming from the Kþ ! mþ n decay; measure the pion crossing time with a resolution of the order of 100 ps to avoid that, due to the high incoming particle rate, the pion track be wrongly associated to a kaon candidate in the GIGATRACKER. produce the level 0 trigger for a charged track.
a magnetic spectrometer (STRAW) measuring the direction of the out-going pion and its momentum;
a photon veto system composed of a set of ring-shaped anti-
counters (LAV) for photons originating from the decay region with angles as large as 48 mrad, a high-performance liquid krypton calorimeter (LKR) for photons in the forward region and finally two calorimeters to veto photons in the angular region around and in the beam (IRC and SAC); a gas Ring Imaging Cherenkov Counter (RICH) providing pion/ muon separation with a muon suppression factor of at least 102 ; a hadron calorimeter and muon detector able to identify muons with very small inefficiencies (MUD).
E-mail address: francesca.bucci@fi.infn.it 0168-9002/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.07.092
2.1. Parameters In the NA62 experiment the RICH is planned to be placed between the fourth STRAW chamber and the liquid krypton calorimeter. The RICH is composed of a cylindrical vessel 18 m long with a diameter of 2.8 m with the beam pipe passing through it. The vessel is filled with neon at atmospheric pressure, which has a refractive index ðn 1Þ ¼ 62:8 106 at l ¼ 300 nm and 25 C. In this case, the maximum angle of Cherenkov photon emission is ymax ¼ 11:2 mrad and the p pion momentum threshold ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi for Cherenkov radiation is pth ¼ mp = ðn2 1Þ ¼ 12 GeV=c. Two semispherical mirrors are placed at the downstream end of the
ARTICLE IN PRESS 48
F. Bucci / Nuclear Instruments and Methods in Physics Research A 595 (2008) 47–50
m 2 ANTI 1-12
M
RICH
HAC MUV
Target
TAX C5
C2
Achromat 2 C7
0
SAC
1
K+∼75 GeV VACUUM
Achromar 1
CEDAR
-1
Ne 1 atm
GIGATRACKER ANTI0
IRC Straw Chambers
-2 0
50
100
150
200
Iron LKr
250
Z m
Fig. 1. NA62 layout.
NPMT e
μ
Fig. 2. The compact hexagonal packing; d is the separation of neighbouring circles’ centers.
vessel, one with the center of curvature 1 m above and the other below the beam axis. The Cherenkov photons are reflected onto photon detectors placed at the focal plane of the mirrors. The focal length of the mirrors is 17 m. The two focal regions are equipped with 1000 photomultipliers (PMTs) each with a pixel size of 18 mm and compact hexagonal packing (Fig. 2). The active area of each PMT is about 8 mm across and a Winston Cone [3–6] is used to convey the Cherenkov light from the 18 mm pixel size. The PMTs are separated from the neon gas by quartz windows. Assuming a mirror reflectivity mirr ¼ 85%,1 a photon detector geometrical acceptance geom ¼ 90%, a funnel collection efficiency coll ¼ 85%, a quartz window transmittance transm ¼ 90% and a quantum efficiency integrated over the photon energy range R q ðEÞ dE 0:6 eV, we estimate the number of hit PMTs (Fig. 3), the Cherenkov angle resolution (Fig. 4) and the pion-muon separation (Fig. 5). In Fig. 5 the squared reconstructed mass m2rec ¼ 2 2 p2 ðymax yC Þ is shown for three different momenta, where p is the momentum reconstructed by the spectrometer. The electron, muon and pion peaks are visible. The muon suppression factor and the pion loss is determined by placing a cut half-way between the muon and the pion peak (Fig. 6). The muon suppression factor in pion selection, integrated over the momentum range from 15 to 35 GeV=c, is 1:3 103 .
1 From the SELEX experiment result for l4160 nm, with a mirror coated with Al and MgF2 .
π
Momentum (GeV/c) Fig. 3. Average number of hit PMTs per event as a function of momentum for pions, muons and electrons from kaon decays.
2.2. Mirrors Since the area to be covered by each semispherical mirror is very large ð2 m2 Þ, a mosaic of smaller segments will be used. Assuming 1 m diameter mirrors, two possibilities are currently under study: using mirrors with hexagonal shape, or mirrors with octagonal and square shapes. Each mirror has a glass bulk 2:5 cm thick, a focal length of 17 m and d0 1 mm.2
2 d0 is the diameter of the circle containing 95% of the imaged light from a point source placed at the center of curvature.
ARTICLE IN PRESS F. Bucci / Nuclear Instruments and Methods in Physics Research A 595 (2008) 47–50
Δθc (rad)
49
μ surv.prob. in e sample e loss
e μ π
Momentum (GeV/c) μ surv.prob. in π sample π loss
Momentum (GeV/c)
Momentum (GeV/c)
Fig. 6. Top plot: electron loss and muon suppression in the electron selection. Bottom plot: pion loss and muon suppression in the pion selection.
Fig. 4. Cherenkov angle resolution as a function of momentum for pions, muons and electrons from kaon decays.
p = 15 GeV/c e
μ
π
reflected light, refocused near to the point-like source, was detected by a photodiode. The focal spot dimension was measured by performing a profile scan at the best focus position with a scan step of 0.1 mm. We measured d0 ¼ 0:5 mm and a focal length f ¼ 17:07 0:01 m. Additional tests (Ronchi and Focault) have confirmed the high optical quality of the mirror. 2.3. Photon detection
mrec2 (GeV2/c4) p = 25 GeV/c e
μ
π
mrec2 (GeV2/c4) p = 35 GeV/c e
μ
π
mrec2 (GeV2/c4) Fig. 5. Reconstructed mass-squared for three different momenta. The peaks of electrons, muons and pions are clearly visible.
The Cherenkov photons reflected by the mirrors are detected by 2000 Hamamatsu PMTs R7400U-03, which are metal package PMTs with an active area of 8 mm diameter. They have an UV-glass window and are sensitive to light with wavelength between 180 and 650 nm. Their nominal gain is 7:0 105 and their transit time spread, defined as the FWHM of the response time, is 280 ps. The quantum efficiency q 20% at l ¼ 410 nm R and q ðEÞ dE 0:6 eV. We have tested almost 100 PMTs, 80 R7400U-03 and 20 R7400U-06. The latter have a quartz window and are sensitive to light down to 160 nm. We illuminated the PMTs with a laser light attenuated by optical filters and spread by a divergent lens to work in single photo-electron conditions. The FWHM of the PMT signal time minus the laser trigger time is 470 ps. After correcting for slewing in the PMT time the FWHM is reduced to 270 ps. The electronics contribution is about 120 ps and that of the laser is negligible. By averaging over 20 PMTs (the average number of hit PMTs), we can reasonably expect an event time resolution better than 100 ps.
3. RICH prototype The quality and the focal length of a circular mirror produced by the MARCON company (San Dona` di Piave, Italy) with a diameter of 50 cm and a glass bulk 2.5 cm thick have been measured. A He–Ne laser ðl ¼ 632:8 nmÞ was placed at the center of curvature of the mirror, slightly off axis. The laser beam was focused and then filtered by a 25 mm diameter pinhole. The
Between the end of October 2007 and the first two weeks of November 2007 a RICH prototype will be exposed to a 200 GeV=c momentum negative beam, mainly composed of pions. A stainless steel vessel, vacuum resistant, 17 m long and with a diameter of 60 cm has already been placed in the NA62 cavern. A 2.5 cm-thick
ARTICLE IN PRESS 50
F. Bucci / Nuclear Instruments and Methods in Physics Research A 595 (2008) 47–50
Fig. 7. Downstream end of the vessel. The mirror is located in the cradle.
circular mirror with a diameter of 50 cm and a focal length of 17 m, built by MARCON, will be used. The mirror is located in a cradle (Fig. 7) at the downstream end of the vessel and can be rotated about two axes by means of two remotely controlled step motors. The vessel will be filled with neon at 980 mbar. The temperature will be monitored by means of six sensors placed along the vessel which will be covered with an insulating foam. At the upstream end of the vessel 96 PMTs are located in the honeycomb positions where the reflected Cherenkov rings are expected (Fig. 8). The test will tell whether the detector performances in terms of Cherenkov angle resolution, number of photoelectrons and time resolution agree with the expectations.
Fig. 8. Upstream end of the vessel. The 96 PMTs and the front-end electronics are visible.
References [1] G. Anelli et al., Proposal to measure the rare decay Kþ ! pþ nn at the CERN SPS, CERN-SPS-2005-013 and CERN-SPSC-P-326, 2005. [2] NA62/P-326 Status Report, CERN-SPSC-2007-035. [3] R. Winston, Light collection within the framework of geometric optics, J. Opt. Soc. Am. 60 (1970) 245–247. [4] R.H. Hildebrand, R. Winston, Throughput of Diffraction-limited field optics system for infrared and millimetric telescopes, Appl. Opt. 21 (1982) 1844–1846. [5] R.H. Hildebrand, Erratum to throughput of diffraction-limited field optics system for infrared and millimetric telescopes, Appl. Opt. 24 (1985) 616. [6] W.T. Welford, R. Winston, High Collection Nonimaging Optics. San Diego: Academic Press, 1989.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A RICH detector for the NA62 very rare kaon decay experiment at CERN F. Bucci ` di Firenze e Sezione dell’INFN di Firenze, via G. Sansone 1, I-50019 Sesto F.(FI), Italy Universita
a r t i c l e in fo
abstract
Available online 20 July 2008
The NA62 experiment is designed to measure the very rare kaon decay Kþ ! pþ nn at the CERN SPS with a statistical precision of less than 10%. The Standard Model prediction for the branching ratio is ð8 1Þ 1011 . One of the challenging aspects of the experiment is the suppression of the Kþ ! mþ nm background at the 1012 level. To satisfy this requirement we need a Ring Imaging Cherenkov Detector able to separate p from m in the momentum range between 15 and 35 GeV=c with a muon suppression factor of at least 102 . We discuss the design and the construction of a RICH detector with a very long focal length ð17 mÞ, neon radiator gas at atmospheric pressure and 2000 fast photodetectors. With a total time resolution of the order of 100 ps this RICH detector can also be used to measure the arrival time of the pion and provide the experiment’s trigger. & 2008 Elsevier B.V. All rights reserved.
Keywords: RICH PID
1. The NA62 experiment The NA62 experiment is designed to measure the Kþ ! pþ nn branching fraction at the CERN Super Proton Synchrotron (SPS). The Standard Model prediction for the branching ratio is ð8 1Þ 1011 . The experiment aims to collect in two years of data taking about 80 Kþ ! pþ nn events with a signal to background ratio S=B ’ 10. We propose to employ a 75 GeV=c momentum beam of positive kaons to be derived from a flux of 400 GeV=c protons. The NA62 detector (Fig. 1) consists of:
A detailed description of the NA62 layout can be found in Refs. [1,2].
2. RICH The RICH detector must fulfill the following requirements:
separate p from m in the momentum range from 15 to
a differential Cherenkov counter (CEDAR) placed in the incoming beam to tag kaons;
thin silicon micro-pixel detectors (GIGATRACKER) able to work at 1 GHz to measure the momentum of the incoming particle;
a set of ring anticounters (CHANTI) to veto charged particles coming from the collimator;
35 GeV=c with a muon suppression factor of at least 102 to help suppressing the background coming from the Kþ ! mþ n decay; measure the pion crossing time with a resolution of the order of 100 ps to avoid that, due to the high incoming particle rate, the pion track be wrongly associated to a kaon candidate in the GIGATRACKER. produce the level 0 trigger for a charged track.
a magnetic spectrometer (STRAW) measuring the direction of the out-going pion and its momentum;
a photon veto system composed of a set of ring-shaped anti-
counters (LAV) for photons originating from the decay region with angles as large as 48 mrad, a high-performance liquid krypton calorimeter (LKR) for photons in the forward region and finally two calorimeters to veto photons in the angular region around and in the beam (IRC and SAC); a gas Ring Imaging Cherenkov Counter (RICH) providing pion/ muon separation with a muon suppression factor of at least 102 ; a hadron calorimeter and muon detector able to identify muons with very small inefficiencies (MUD).
E-mail address: francesca.bucci@fi.infn.it 0168-9002/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.07.092
2.1. Parameters In the NA62 experiment the RICH is planned to be placed between the fourth STRAW chamber and the liquid krypton calorimeter. The RICH is composed of a cylindrical vessel 18 m long with a diameter of 2.8 m with the beam pipe passing through it. The vessel is filled with neon at atmospheric pressure, which has a refractive index ðn 1Þ ¼ 62:8 106 at l ¼ 300 nm and 25 C. In this case, the maximum angle of Cherenkov photon emission is ymax ¼ 11:2 mrad and the p pion momentum threshold ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi for Cherenkov radiation is pth ¼ mp = ðn2 1Þ ¼ 12 GeV=c. Two semispherical mirrors are placed at the downstream end of the
ARTICLE IN PRESS 48
F. Bucci / Nuclear Instruments and Methods in Physics Research A 595 (2008) 47–50
m 2 ANTI 1-12
M
RICH
HAC MUV
Target
TAX C5
C2
Achromat 2 C7
0
SAC
1
K+∼75 GeV VACUUM
Achromar 1
CEDAR
-1
Ne 1 atm
GIGATRACKER ANTI0
IRC Straw Chambers
-2 0
50
100
150
200
Iron LKr
250
Z m
Fig. 1. NA62 layout.
NPMT e
μ
Fig. 2. The compact hexagonal packing; d is the separation of neighbouring circles’ centers.
vessel, one with the center of curvature 1 m above and the other below the beam axis. The Cherenkov photons are reflected onto photon detectors placed at the focal plane of the mirrors. The focal length of the mirrors is 17 m. The two focal regions are equipped with 1000 photomultipliers (PMTs) each with a pixel size of 18 mm and compact hexagonal packing (Fig. 2). The active area of each PMT is about 8 mm across and a Winston Cone [3–6] is used to convey the Cherenkov light from the 18 mm pixel size. The PMTs are separated from the neon gas by quartz windows. Assuming a mirror reflectivity mirr ¼ 85%,1 a photon detector geometrical acceptance geom ¼ 90%, a funnel collection efficiency coll ¼ 85%, a quartz window transmittance transm ¼ 90% and a quantum efficiency integrated over the photon energy range R q ðEÞ dE 0:6 eV, we estimate the number of hit PMTs (Fig. 3), the Cherenkov angle resolution (Fig. 4) and the pion-muon separation (Fig. 5). In Fig. 5 the squared reconstructed mass m2rec ¼ 2 2 p2 ðymax yC Þ is shown for three different momenta, where p is the momentum reconstructed by the spectrometer. The electron, muon and pion peaks are visible. The muon suppression factor and the pion loss is determined by placing a cut half-way between the muon and the pion peak (Fig. 6). The muon suppression factor in pion selection, integrated over the momentum range from 15 to 35 GeV=c, is 1:3 103 .
1 From the SELEX experiment result for l4160 nm, with a mirror coated with Al and MgF2 .
π
Momentum (GeV/c) Fig. 3. Average number of hit PMTs per event as a function of momentum for pions, muons and electrons from kaon decays.
2.2. Mirrors Since the area to be covered by each semispherical mirror is very large ð2 m2 Þ, a mosaic of smaller segments will be used. Assuming 1 m diameter mirrors, two possibilities are currently under study: using mirrors with hexagonal shape, or mirrors with octagonal and square shapes. Each mirror has a glass bulk 2:5 cm thick, a focal length of 17 m and d0 1 mm.2
2 d0 is the diameter of the circle containing 95% of the imaged light from a point source placed at the center of curvature.
ARTICLE IN PRESS F. Bucci / Nuclear Instruments and Methods in Physics Research A 595 (2008) 47–50
Δθc (rad)
49
μ surv.prob. in e sample e loss
e μ π
Momentum (GeV/c) μ surv.prob. in π sample π loss
Momentum (GeV/c)
Momentum (GeV/c)
Fig. 6. Top plot: electron loss and muon suppression in the electron selection. Bottom plot: pion loss and muon suppression in the pion selection.
Fig. 4. Cherenkov angle resolution as a function of momentum for pions, muons and electrons from kaon decays.
p = 15 GeV/c e
μ
π
reflected light, refocused near to the point-like source, was detected by a photodiode. The focal spot dimension was measured by performing a profile scan at the best focus position with a scan step of 0.1 mm. We measured d0 ¼ 0:5 mm and a focal length f ¼ 17:07 0:01 m. Additional tests (Ronchi and Focault) have confirmed the high optical quality of the mirror. 2.3. Photon detection
mrec2 (GeV2/c4) p = 25 GeV/c e
μ
π
mrec2 (GeV2/c4) p = 35 GeV/c e
μ
π
mrec2 (GeV2/c4) Fig. 5. Reconstructed mass-squared for three different momenta. The peaks of electrons, muons and pions are clearly visible.
The Cherenkov photons reflected by the mirrors are detected by 2000 Hamamatsu PMTs R7400U-03, which are metal package PMTs with an active area of 8 mm diameter. They have an UV-glass window and are sensitive to light with wavelength between 180 and 650 nm. Their nominal gain is 7:0 105 and their transit time spread, defined as the FWHM of the response time, is 280 ps. The quantum efficiency q 20% at l ¼ 410 nm R and q ðEÞ dE 0:6 eV. We have tested almost 100 PMTs, 80 R7400U-03 and 20 R7400U-06. The latter have a quartz window and are sensitive to light down to 160 nm. We illuminated the PMTs with a laser light attenuated by optical filters and spread by a divergent lens to work in single photo-electron conditions. The FWHM of the PMT signal time minus the laser trigger time is 470 ps. After correcting for slewing in the PMT time the FWHM is reduced to 270 ps. The electronics contribution is about 120 ps and that of the laser is negligible. By averaging over 20 PMTs (the average number of hit PMTs), we can reasonably expect an event time resolution better than 100 ps.
3. RICH prototype The quality and the focal length of a circular mirror produced by the MARCON company (San Dona` di Piave, Italy) with a diameter of 50 cm and a glass bulk 2.5 cm thick have been measured. A He–Ne laser ðl ¼ 632:8 nmÞ was placed at the center of curvature of the mirror, slightly off axis. The laser beam was focused and then filtered by a 25 mm diameter pinhole. The
Between the end of October 2007 and the first two weeks of November 2007 a RICH prototype will be exposed to a 200 GeV=c momentum negative beam, mainly composed of pions. A stainless steel vessel, vacuum resistant, 17 m long and with a diameter of 60 cm has already been placed in the NA62 cavern. A 2.5 cm-thick
ARTICLE IN PRESS 50
F. Bucci / Nuclear Instruments and Methods in Physics Research A 595 (2008) 47–50
Fig. 7. Downstream end of the vessel. The mirror is located in the cradle.
circular mirror with a diameter of 50 cm and a focal length of 17 m, built by MARCON, will be used. The mirror is located in a cradle (Fig. 7) at the downstream end of the vessel and can be rotated about two axes by means of two remotely controlled step motors. The vessel will be filled with neon at 980 mbar. The temperature will be monitored by means of six sensors placed along the vessel which will be covered with an insulating foam. At the upstream end of the vessel 96 PMTs are located in the honeycomb positions where the reflected Cherenkov rings are expected (Fig. 8). The test will tell whether the detector performances in terms of Cherenkov angle resolution, number of photoelectrons and time resolution agree with the expectations.
Fig. 8. Upstream end of the vessel. The 96 PMTs and the front-end electronics are visible.
References [1] G. Anelli et al., Proposal to measure the rare decay Kþ ! pþ nn at the CERN SPS, CERN-SPS-2005-013 and CERN-SPSC-P-326, 2005. [2] NA62/P-326 Status Report, CERN-SPSC-2007-035. [3] R. Winston, Light collection within the framework of geometric optics, J. Opt. Soc. Am. 60 (1970) 245–247. [4] R.H. Hildebrand, R. Winston, Throughput of Diffraction-limited field optics system for infrared and millimetric telescopes, Appl. Opt. 21 (1982) 1844–1846. [5] R.H. Hildebrand, Erratum to throughput of diffraction-limited field optics system for infrared and millimetric telescopes, Appl. Opt. 24 (1985) 616. [6] W.T. Welford, R. Winston, High Collection Nonimaging Optics. San Diego: Academic Press, 1989.