- Email: [email protected]
A new aerogel Cherenkov detector for DIRAC-II
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 595 (2008) 84–87
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A new aerogel Cherenkov detector for DIRAC-II Y. Allkofer, C. Amsler, S. Horikawa , C. Regenfus, J. Rochet ¨t Zu ¨ rich, CH-8057 Zu ¨ rich, Switzerland Physik-Institut der Universita
a r t i c l e in f o
a b s t r a c t
Available online 17 July 2008
A new threshold Cherenkov detector using silica aerogel radiators is in operation in the DIRAC-II experiment at CERN. A duplex design was chosen to discriminate between kaons and protons in the momentum range 4–8 GeV/c. The counter consists of overlapping modules with two different refractive indices, 1.015 and 1.008, covering the low and high momentum regions, respectively. We developed a new design of alternating layers of aerogel tiles and diffusive reflector foils coated with wavelength shifter to improve light collection in the aerogel. A pyramidal shape of the aerogel radiator was also adopted for best signal uniformity over a large sensitive area. We report on the detector status and its performance in the DIRAC 2007 run. & 2008 Elsevier B.V. All rights reserved.
Keywords: Aerogel Wavelength shifter Particle identification
1. Introduction—DIRAC-II DIRAC-II is a fixed target experiment at the CERN-PS involving about 80 physicists from seven countries [1]. Its physics goals are to measure the lifetime of pp and pK atoms and consequently to determine the associating scattering lengths in a model independent way. Similar to the pp atoms, the lifetime t of pK atoms can be expressed with the isospins 12 and 32 S-wave pK scattering lengths, a1 and a3 , respectively, 1
t
¼
8a3 2 m p ja1 a3 j2 ð1 þ dÞ 9
(1)
where a is the fine structure constant, m the reduced mass of the pK system, p the momentum of p0 ðK0 Þ from the decay and d a correction term. The scattering length is one of the observables which can be calculated with highest precision using low-energy QCD, chiral perturbation theory (ChPT). Hence, it can be considered as an excellent tool to test the theory. DIRAC in its first phase measured the pp-atom lifetime. The result t ¼ 2:91þ0:49 0:62 fs [2] is in very good agreement with the ChPT prediction, t ¼ 2:9 0:1 fs [3]. The pK scattering lengths are of greater interest because one can test ChPT including s-quarks. In contrast to the pp case, however, the knowledge is very limited. The last experimental results obtained from scattering experiments in the 70s are inconsistent and in contradiction with the theoretical predictions [4]. Precise measurements of the pK scattering lengths are thus highly desirable. DIRAC has been upgraded to DIRAC-II to measure pK atoms for the first time. Its experimental approach is well Corresponding author.
E-mail address: [email protected] (S. Horikawa). 0168-9002/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.07.046
established in the successful pp measurements, and therefore is very promising. DIRAC-II is aiming at the determination of the pK-atom lifetime with 20% precision, which leads to 10% precision on ja1 a3 j. The DIRAC-II spectrometer is schematically illustrated in Fig. 1. It is a double-arm magnetic spectrometer in which the momentum vectors of two oppositely charged hadrons are measured. In DIRAC-II, p Kþ (or pþ K ) atoms are produced in a target, typically a 100 mm thick nickel foil, by the 24 GeV/c proton beam. The atoms annihilate into p0 K0 (or p0 K¯ 0 ) or ionise (breakup) in the target. The mean life can be determined by measuring the number of dissociated pK pairs. The key feature of the upgraded spectrometer is kaon identification, which is realised by the two threshold Cherenkov detectors. The new heavy-gas Cherenkov detectors running on C4F10 gas (n1:00137) [5] detect only pions, while kaons are detected by the aerogel Cherenkov detector (n ¼ 1:015 and 1.008). Contaminating protons in the Kþ -arm are suppressed since they are below threshold for Cherenkov radiation.
2. Detector design Fig. 2 shows the distribution of kaons from the pK-atom breakup in the DIRAC-II apparatus. Having momenta ranging between 4 and 8 GeV/c kaons are distributed over a distance of 35 cm in the horizontal plane due to the deflection angle in the dipole magnet. The boxes with numbers 1–3 show the regions covered by the corresponding modules illustrated in Fig. 3. The detector consists of three geometrically identical modules with two different refractive indices. The light-gel (n ¼ 1:008) module (3) is overlapping with one of the heavy-gel (n ¼ 1:015) modules (2) covering the high momentum kaons above 5.5 GeV/c.
ARTICLE IN PRESS Y. Allkofer et al. / Nuclear Instruments and Methods in Physics Research A 595 (2008) 84–87
85
heavy-gas Cherenkov SFD
MDC, 18 planes
aerogel
IH
T1
single or multilayer target absorber
MDC IH P vacuum
1 meter
SFD
vacuum
shield1 shield2
magnet
DC VH HH
T2
24 GeV/c protons on Ni f oil target (100 μm) Ch PSh
Mu
Fig. 1. The DIRAC-II spectrometer.
10
34 cm
Kaon momentum (GeV/c)
large angle
small angle
17 cm
PMTs
8
n = 1.008 3
3
6
n = 1.015 1
2
40 cm 1
2
4
large angle
40
50
60
70
80
π , K, p
90
Horizontal coordinate (cm) Fig. 2. Distribution of kaons from the breakup of pK atoms. The boxes with numbers 1–3 show the regions covered by the corresponding modules in Fig. 3.
The design, in which 5-in. PMTs (Photonis XP4570/B with UV-glass, selected for cathode blue sensitivity larger than 11 mA=lm) are directly mounted on the top and on the bottom surfaces of the radiator box (40 cm high), was optimised for the maximal efficiency in limited space. The major challenges during detector development were to improve (1) the light yield and (2) the uniformity of the detector response over the large surface. The very small refractive indices of aerogel lead to a small number of Cherenkov photons. In addition, the large distance between the PMTs results in a strong dependence of the detector response on the (vertical) impact coordinate due to significant absorption in the aerogel. These stringent conditions are circumvented by introducing two additional features. Fig. 4 illustrates the sandwich design incorporating a wavelength shifter (WLS) to increase the collection and detection efficiency for UV light. Tetratex reflector foils coated with WLS
small angle
Fig. 3. Sketch of the DIRAC aerogel detector. Modules 1 and 2 (n ¼ 1:015) cover the momentum range 4–5.5 GeV/c and module 3 (n ¼ 1:008) covers the range 5.5–8 GeV/c. The PMTs are mounted on the top and bottom surfaces of each module.
Incident particles
Tetratex +WLS
Aerogel
Fig. 4. Sketch of the sandwich design.
and layers of aerogel radiator are alternately stacked in the direction of incident particles. The naturally more abundant UV photons of the Cherenkov radiation are converted to longer wavelengths by the WLS. This provides a better matching to the PMT sensitivity and also takes advantage of the larger absorption length of aerogel, which increases rapidly from about 10 cm at
ARTICLE IN PRESS 86
Y. Allkofer et al. / Nuclear Instruments and Methods in Physics Research A 595 (2008) 84–87
270 nm to 3 m at 350 nm [6]. The sandwich design was tested using cosmic rays on a small detector prototype with aerogel n ¼ 1:05 [7]. Various WLS materials were tested, such as POPOP, tetraphenylbutadiene (TPB) and p-terphenyl (pT). Different techniques were investigated to deposit the WLS: (A) dissolving the WLS in a solvent and spraying the solution on the foil, (B)
pT (86470)
50 Only aerogel
Npe
40
30 TPB (88020) 20 Other WLS 10
POPOP (15150, 15080)
0
immersing the foil in the solution (A). Fig. 5 shows the average number of photoelectrons for various configurations (different solvents and coating techniques). The best result, 50% increase in yield, was obtained with pT dissolved in chloroform and with the immersing technique. Note that Tetratex foils with WLS were also tested without aerogel but no light yield could be observed within the measurement precision. Evaporation of pT on the foil was also tested but did not give satisfactory results because of poor adherence of pT to Tetratex. The three-layer sandwich design with pT and immersing technique is implemented in the light-gel module, for which the light yield is more crucial. To compensate for the strong position dependence of the light yield, aerogel tiles are added at the centre of the detector forming a unique pyramidal shape as seen in Fig. 6. A three step pyramid which reduces the variation of the number of photoelectrons over the aerogel length from 40% to 15% (according to Monte Carlo simulation) is implemented in every module. Signal uniformity within 10% was confirmed in the final detector in cosmic ray tests and also during the first DIRAC-II runs. Fig. 7 shows an overview of the final detector built in 2006. After extensive studies using cosmic rays and simulations, we chose the aerogel with n ¼ 1:008 from the Budker Institute for Nuclear Physics (BINP) associated with the Boreskov Institute of Catalysis (BIC) in Novosibirsk [8], and the one with n ¼ 1:015 from Matsushita (Panasonic) Electric Works (MEW) Ltd. in Japan [9]. The detector design and the R&D results are described in detail in Ref. [7].
Configuration Fig. 5. Comparison of light yield (number of photoelectrons N pe ) in the sandwich design with n ¼ 1:05 aerogel for various configurations (WLS, solvents and coating techniques).
3. In-beam performance The detector is operating since June 2007 in the DIRAC-II beam and a preliminary performance analysis has been done. Fig. 8 shows a pion amplitude spectrum obtained from the aerogel detector (heavy-gel module, n ¼ 1:015). Positively charged tracks crossing the detectors with momenta above 4 GeV/c were selected using tracking detectors. A signal above pedestal in the C4F10 detector is required in coincidence to select pions. The clear separation from pedestal indicates a high detection efficiency. A large part of the pedestal in Fig. 8 is due to the noise in the trigger or in the C4F10 detector.
350 Pedestal 300 250 Number of events
Fig. 6. Pyramidal shape of the aerogel radiator (n ¼ 1:015).
200 150 100 50
π, K, p 0
Fig. 7. Overview of the final set-up.
100
200 300 Amplitude
400
500
Fig. 8. Amplitude spectrum for pions above 4 GeV/c obtained from the aerogel detector (heavy-gel module, n ¼ 1:015).
ARTICLE IN PRESS Y. Allkofer et al. / Nuclear Instruments and Methods in Physics Research A 595 (2008) 84–87
50
100
n = 1.008
n = 1.015 80
Number of events
Number of events
87
4 < p < 5.2 GeV/c 60 Npe ~ 6.9 40
40 p > 6 GeV/c 30 Npe ~ 3.9 20 10
20
0
5
10
15 Npe
20
25
30
0
4
8
12
16
20
Npe
Fig. 9. Kaon spectra in the aerogel detector. Left: n ¼ 1:015, right: n ¼ 1:008. The signal amplitude is normalised to the number of photoelectrons N pe .
Kaon spectra in the aerogel detector can in principle be obtained using the C4F10 detector in anti-coincidence. However, the signal was not clearly separated from pedestal because of noise in coincidence with copious protons. To get clean kaon spectra we inverted the polarity of the spectrometer dipole magnet so that negative particles could cross the detector. The two plots in Fig. 9 are thus K spectra from the heavy- and lightgel modules. The signal amplitude is normalised to the number of photoelectrons N pe by the single-photoelectron spectrum. The pedestal counts agree with the expectation for contaminating x antiprotons. With a Poisson fit PðxÞ ¼ C l el =Gðx þ 1Þ with C and l as free parameters the average number of detected photoelectrons is N e ¼ l6:9 and 3.9 for heavy- and light-gel modules, respectively. We used the K spectra to estimate the kaon detection efficiency, while the proton rejection factor was studied with Kþ spectra applying the same cuts. After a preliminary optimisation of cuts, we obtained a kaon (Kþ ) detection efficiency of 85–95% with a proton rejection factor of 25–50. These numbers are very preliminary and intense analyses are ongoing for a precise understanding of the detector response, which is essential for the determination of the atom lifetime.
4. Conclusions and outlook An aerogel threshold Cherenkov detector has been built introducing new features: (1) a sandwich design using a WLS to improve the light yield and (2) a pyramid shape to improve the signal uniformity. The device is in operation in DIRAC-II since June 2007. A preliminary performance analysis demonstrated a kaon
detection efficiency of 85–95% with a proton rejection factor of 25–50. Improvements in the trigger and readout electronics should further improve these factors.
Acknowledgements The authors gratefully acknowledge the support of technical staffs at the University of Zurich and at CERN. They thank E.A. Kravchenko and A.P. Onuchin of the Budker Institute for Nuclear Physics, and A.F. Danilyuk of the Boreskov Institute of Catalysis for fruitful discussions. They are also indebted to members of the DIRAC-II Collaboration for their support. This work was supported by the Swiss National Science Foundation. References [1] Y. Allkofer, et al., Proceedings of the XII Conference on Hadron Spectroscopy (Hadron07), Frascati, 2007, in print. [2] B. Adeva, DIRAC Collaboration, et al., Phys. Lett. B 619 (2005) 50; B. Adeva, DIRAC Collaboration, et al., J. Phys. G 30 (2004) 1929. [3] G. Colangelo, J. Gasser, H. Leutwyler, Nucl. Phys. B 603 (2001) 125 [arXiv: hep-ph/0103088]. [4] V. Bernard, N. Kaiser, U.G. Meissner, Nucl. Phys. B 357 (1991) 129; P. Buettiker, S. Descotes-Genon, B. Moussallam, Eur. Phys. J. C 33 (2004) 409. [5] S. Horikawa, et al., The C4F10 Cherenkov detector for DIRAC-II, Nucl. Instr. and Meth. A, 2008, this issue, doi:10.1016/j.nima.2008.07.047. [6] A.F. Danilyuk, et al., Nucl. Instr. and Meth. A 494 (2002) 491. [7] Y. Allkofer, et al., Nucl. Instr. and Meth. 582 (2007) 497. [8] A.Y. Barnyakov, et al., Nucl. Instr. and Meth. A 553 (2005) 125. [9] Matsushita Electric Works Ltd., 1048 Kadoma, Kadoma-shi, Osaka 571-8686, Japan hhttp://www.mew.co.jp/e-aerogel/i.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A new aerogel Cherenkov detector for DIRAC-II Y. Allkofer, C. Amsler, S. Horikawa , C. Regenfus, J. Rochet ¨t Zu ¨ rich, CH-8057 Zu ¨ rich, Switzerland Physik-Institut der Universita
a r t i c l e in f o
a b s t r a c t
Available online 17 July 2008
A new threshold Cherenkov detector using silica aerogel radiators is in operation in the DIRAC-II experiment at CERN. A duplex design was chosen to discriminate between kaons and protons in the momentum range 4–8 GeV/c. The counter consists of overlapping modules with two different refractive indices, 1.015 and 1.008, covering the low and high momentum regions, respectively. We developed a new design of alternating layers of aerogel tiles and diffusive reflector foils coated with wavelength shifter to improve light collection in the aerogel. A pyramidal shape of the aerogel radiator was also adopted for best signal uniformity over a large sensitive area. We report on the detector status and its performance in the DIRAC 2007 run. & 2008 Elsevier B.V. All rights reserved.
Keywords: Aerogel Wavelength shifter Particle identification
1. Introduction—DIRAC-II DIRAC-II is a fixed target experiment at the CERN-PS involving about 80 physicists from seven countries [1]. Its physics goals are to measure the lifetime of pp and pK atoms and consequently to determine the associating scattering lengths in a model independent way. Similar to the pp atoms, the lifetime t of pK atoms can be expressed with the isospins 12 and 32 S-wave pK scattering lengths, a1 and a3 , respectively, 1
t
¼
8a3 2 m p ja1 a3 j2 ð1 þ dÞ 9
(1)
where a is the fine structure constant, m the reduced mass of the pK system, p the momentum of p0 ðK0 Þ from the decay and d a correction term. The scattering length is one of the observables which can be calculated with highest precision using low-energy QCD, chiral perturbation theory (ChPT). Hence, it can be considered as an excellent tool to test the theory. DIRAC in its first phase measured the pp-atom lifetime. The result t ¼ 2:91þ0:49 0:62 fs [2] is in very good agreement with the ChPT prediction, t ¼ 2:9 0:1 fs [3]. The pK scattering lengths are of greater interest because one can test ChPT including s-quarks. In contrast to the pp case, however, the knowledge is very limited. The last experimental results obtained from scattering experiments in the 70s are inconsistent and in contradiction with the theoretical predictions [4]. Precise measurements of the pK scattering lengths are thus highly desirable. DIRAC has been upgraded to DIRAC-II to measure pK atoms for the first time. Its experimental approach is well Corresponding author.
E-mail address: [email protected] (S. Horikawa). 0168-9002/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.07.046
established in the successful pp measurements, and therefore is very promising. DIRAC-II is aiming at the determination of the pK-atom lifetime with 20% precision, which leads to 10% precision on ja1 a3 j. The DIRAC-II spectrometer is schematically illustrated in Fig. 1. It is a double-arm magnetic spectrometer in which the momentum vectors of two oppositely charged hadrons are measured. In DIRAC-II, p Kþ (or pþ K ) atoms are produced in a target, typically a 100 mm thick nickel foil, by the 24 GeV/c proton beam. The atoms annihilate into p0 K0 (or p0 K¯ 0 ) or ionise (breakup) in the target. The mean life can be determined by measuring the number of dissociated pK pairs. The key feature of the upgraded spectrometer is kaon identification, which is realised by the two threshold Cherenkov detectors. The new heavy-gas Cherenkov detectors running on C4F10 gas (n1:00137) [5] detect only pions, while kaons are detected by the aerogel Cherenkov detector (n ¼ 1:015 and 1.008). Contaminating protons in the Kþ -arm are suppressed since they are below threshold for Cherenkov radiation.
2. Detector design Fig. 2 shows the distribution of kaons from the pK-atom breakup in the DIRAC-II apparatus. Having momenta ranging between 4 and 8 GeV/c kaons are distributed over a distance of 35 cm in the horizontal plane due to the deflection angle in the dipole magnet. The boxes with numbers 1–3 show the regions covered by the corresponding modules illustrated in Fig. 3. The detector consists of three geometrically identical modules with two different refractive indices. The light-gel (n ¼ 1:008) module (3) is overlapping with one of the heavy-gel (n ¼ 1:015) modules (2) covering the high momentum kaons above 5.5 GeV/c.
ARTICLE IN PRESS Y. Allkofer et al. / Nuclear Instruments and Methods in Physics Research A 595 (2008) 84–87
85
heavy-gas Cherenkov SFD
MDC, 18 planes
aerogel
IH
T1
single or multilayer target absorber
MDC IH P vacuum
1 meter
SFD
vacuum
shield1 shield2
magnet
DC VH HH
T2
24 GeV/c protons on Ni f oil target (100 μm) Ch PSh
Mu
Fig. 1. The DIRAC-II spectrometer.
10
34 cm
Kaon momentum (GeV/c)
large angle
small angle
17 cm
PMTs
8
n = 1.008 3
3
6
n = 1.015 1
2
40 cm 1
2
4
large angle
40
50
60
70
80
π , K, p
90
Horizontal coordinate (cm) Fig. 2. Distribution of kaons from the breakup of pK atoms. The boxes with numbers 1–3 show the regions covered by the corresponding modules in Fig. 3.
The design, in which 5-in. PMTs (Photonis XP4570/B with UV-glass, selected for cathode blue sensitivity larger than 11 mA=lm) are directly mounted on the top and on the bottom surfaces of the radiator box (40 cm high), was optimised for the maximal efficiency in limited space. The major challenges during detector development were to improve (1) the light yield and (2) the uniformity of the detector response over the large surface. The very small refractive indices of aerogel lead to a small number of Cherenkov photons. In addition, the large distance between the PMTs results in a strong dependence of the detector response on the (vertical) impact coordinate due to significant absorption in the aerogel. These stringent conditions are circumvented by introducing two additional features. Fig. 4 illustrates the sandwich design incorporating a wavelength shifter (WLS) to increase the collection and detection efficiency for UV light. Tetratex reflector foils coated with WLS
small angle
Fig. 3. Sketch of the DIRAC aerogel detector. Modules 1 and 2 (n ¼ 1:015) cover the momentum range 4–5.5 GeV/c and module 3 (n ¼ 1:008) covers the range 5.5–8 GeV/c. The PMTs are mounted on the top and bottom surfaces of each module.
Incident particles
Tetratex +WLS
Aerogel
Fig. 4. Sketch of the sandwich design.
and layers of aerogel radiator are alternately stacked in the direction of incident particles. The naturally more abundant UV photons of the Cherenkov radiation are converted to longer wavelengths by the WLS. This provides a better matching to the PMT sensitivity and also takes advantage of the larger absorption length of aerogel, which increases rapidly from about 10 cm at
ARTICLE IN PRESS 86
Y. Allkofer et al. / Nuclear Instruments and Methods in Physics Research A 595 (2008) 84–87
270 nm to 3 m at 350 nm [6]. The sandwich design was tested using cosmic rays on a small detector prototype with aerogel n ¼ 1:05 [7]. Various WLS materials were tested, such as POPOP, tetraphenylbutadiene (TPB) and p-terphenyl (pT). Different techniques were investigated to deposit the WLS: (A) dissolving the WLS in a solvent and spraying the solution on the foil, (B)
pT (86470)
50 Only aerogel
Npe
40
30 TPB (88020) 20 Other WLS 10
POPOP (15150, 15080)
0
immersing the foil in the solution (A). Fig. 5 shows the average number of photoelectrons for various configurations (different solvents and coating techniques). The best result, 50% increase in yield, was obtained with pT dissolved in chloroform and with the immersing technique. Note that Tetratex foils with WLS were also tested without aerogel but no light yield could be observed within the measurement precision. Evaporation of pT on the foil was also tested but did not give satisfactory results because of poor adherence of pT to Tetratex. The three-layer sandwich design with pT and immersing technique is implemented in the light-gel module, for which the light yield is more crucial. To compensate for the strong position dependence of the light yield, aerogel tiles are added at the centre of the detector forming a unique pyramidal shape as seen in Fig. 6. A three step pyramid which reduces the variation of the number of photoelectrons over the aerogel length from 40% to 15% (according to Monte Carlo simulation) is implemented in every module. Signal uniformity within 10% was confirmed in the final detector in cosmic ray tests and also during the first DIRAC-II runs. Fig. 7 shows an overview of the final detector built in 2006. After extensive studies using cosmic rays and simulations, we chose the aerogel with n ¼ 1:008 from the Budker Institute for Nuclear Physics (BINP) associated with the Boreskov Institute of Catalysis (BIC) in Novosibirsk [8], and the one with n ¼ 1:015 from Matsushita (Panasonic) Electric Works (MEW) Ltd. in Japan [9]. The detector design and the R&D results are described in detail in Ref. [7].
Configuration Fig. 5. Comparison of light yield (number of photoelectrons N pe ) in the sandwich design with n ¼ 1:05 aerogel for various configurations (WLS, solvents and coating techniques).
3. In-beam performance The detector is operating since June 2007 in the DIRAC-II beam and a preliminary performance analysis has been done. Fig. 8 shows a pion amplitude spectrum obtained from the aerogel detector (heavy-gel module, n ¼ 1:015). Positively charged tracks crossing the detectors with momenta above 4 GeV/c were selected using tracking detectors. A signal above pedestal in the C4F10 detector is required in coincidence to select pions. The clear separation from pedestal indicates a high detection efficiency. A large part of the pedestal in Fig. 8 is due to the noise in the trigger or in the C4F10 detector.
350 Pedestal 300 250 Number of events
Fig. 6. Pyramidal shape of the aerogel radiator (n ¼ 1:015).
200 150 100 50
π, K, p 0
Fig. 7. Overview of the final set-up.
100
200 300 Amplitude
400
500
Fig. 8. Amplitude spectrum for pions above 4 GeV/c obtained from the aerogel detector (heavy-gel module, n ¼ 1:015).
ARTICLE IN PRESS Y. Allkofer et al. / Nuclear Instruments and Methods in Physics Research A 595 (2008) 84–87
50
100
n = 1.008
n = 1.015 80
Number of events
Number of events
87
4 < p < 5.2 GeV/c 60 Npe ~ 6.9 40
40 p > 6 GeV/c 30 Npe ~ 3.9 20 10
20
0
5
10
15 Npe
20
25
30
0
4
8
12
16
20
Npe
Fig. 9. Kaon spectra in the aerogel detector. Left: n ¼ 1:015, right: n ¼ 1:008. The signal amplitude is normalised to the number of photoelectrons N pe .
Kaon spectra in the aerogel detector can in principle be obtained using the C4F10 detector in anti-coincidence. However, the signal was not clearly separated from pedestal because of noise in coincidence with copious protons. To get clean kaon spectra we inverted the polarity of the spectrometer dipole magnet so that negative particles could cross the detector. The two plots in Fig. 9 are thus K spectra from the heavy- and lightgel modules. The signal amplitude is normalised to the number of photoelectrons N pe by the single-photoelectron spectrum. The pedestal counts agree with the expectation for contaminating x antiprotons. With a Poisson fit PðxÞ ¼ C l el =Gðx þ 1Þ with C and l as free parameters the average number of detected photoelectrons is N e ¼ l6:9 and 3.9 for heavy- and light-gel modules, respectively. We used the K spectra to estimate the kaon detection efficiency, while the proton rejection factor was studied with Kþ spectra applying the same cuts. After a preliminary optimisation of cuts, we obtained a kaon (Kþ ) detection efficiency of 85–95% with a proton rejection factor of 25–50. These numbers are very preliminary and intense analyses are ongoing for a precise understanding of the detector response, which is essential for the determination of the atom lifetime.
4. Conclusions and outlook An aerogel threshold Cherenkov detector has been built introducing new features: (1) a sandwich design using a WLS to improve the light yield and (2) a pyramid shape to improve the signal uniformity. The device is in operation in DIRAC-II since June 2007. A preliminary performance analysis demonstrated a kaon
detection efficiency of 85–95% with a proton rejection factor of 25–50. Improvements in the trigger and readout electronics should further improve these factors.
Acknowledgements The authors gratefully acknowledge the support of technical staffs at the University of Zurich and at CERN. They thank E.A. Kravchenko and A.P. Onuchin of the Budker Institute for Nuclear Physics, and A.F. Danilyuk of the Boreskov Institute of Catalysis for fruitful discussions. They are also indebted to members of the DIRAC-II Collaboration for their support. This work was supported by the Swiss National Science Foundation. References [1] Y. Allkofer, et al., Proceedings of the XII Conference on Hadron Spectroscopy (Hadron07), Frascati, 2007, in print. [2] B. Adeva, DIRAC Collaboration, et al., Phys. Lett. B 619 (2005) 50; B. Adeva, DIRAC Collaboration, et al., J. Phys. G 30 (2004) 1929. [3] G. Colangelo, J. Gasser, H. Leutwyler, Nucl. Phys. B 603 (2001) 125 [arXiv: hep-ph/0103088]. [4] V. Bernard, N. Kaiser, U.G. Meissner, Nucl. Phys. B 357 (1991) 129; P. Buettiker, S. Descotes-Genon, B. Moussallam, Eur. Phys. J. C 33 (2004) 409. [5] S. Horikawa, et al., The C4F10 Cherenkov detector for DIRAC-II, Nucl. Instr. and Meth. A, 2008, this issue, doi:10.1016/j.nima.2008.07.047. [6] A.F. Danilyuk, et al., Nucl. Instr. and Meth. A 494 (2002) 491. [7] Y. Allkofer, et al., Nucl. Instr. and Meth. 582 (2007) 497. [8] A.Y. Barnyakov, et al., Nucl. Instr. and Meth. A 553 (2005) 125. [9] Matsushita Electric Works Ltd., 1048 Kadoma, Kadoma-shi, Osaka 571-8686, Japan hhttp://www.mew.co.jp/e-aerogel/i.