- Email: [email protected]
Study of a water Cherenkov detector for low energy v-e scattering
& ‘r-5 _-
Nuclear Instruments and Methods in Physics Research A 360 (199.5)395-399
NUCLEAR INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH
__ LiiB
SectIonA
ELSEWIER
Study of a water Cherenkov detector for low energy v-e scattering B. Armbruster,
G. Drexlin, V. Eberhard, H. Gemmeke
*, B. Zeitnitz
Forschungszentrum Karlsruhe and Universiq Karlsruhe, Karlsruhe, Germany
Abstract For low energy tests of the standard model by v-e scattering and the determination of v--‘~O cross sections a high resolution 1.3 kt Hz0 Cherenkov detector is under evaluation. The results of extensive Monte Carlo simulations are presented. The goal is a high resolution Cherenkov detector with time resolution of 0.6 ns and energy resolution of 17% at E, = 15 MeV.
1. ~nt~duction We propose to use a water Cherenkov detector to proceed in the sequence of high precision tests of the weak interaction with KARMEN ’ [l]. A water Cherenkov detector is an ideal instrument to study the strongly forward peaked neutrino-electron scattering in the presence of more isotropic distributed reactions, as inverse B-decay on 160 and muon induced background [2]. In addition such a detector is not very sensitive to the low energy neutron background unavoidably produced in a massive beam dump u-source. Furthermore, a water Cherenkov detector has the advantage to determine the hitherto unmeasured v~-‘~O cross section which is important for neutrino astrophysics, whereas a mineral oil based detector as LSND would remeasure the v-t? cross section well known from KARMEN. Neutrino reactions in water cover a wide range of physical topics. Neutrino-electron scattering of the type u,--e allows us to measure precisely the interference of neutral- and charged-current reactions. The measurement of the differential cross section, especially of the VI,--e reaction, allows one to extract the ratio of the left handed and right handed coupling constants, &gf, in a flux independent manner from the shape of the spectrum. The cross section of w-e scattering is strongly enhanced in the low energy region by a hypothetical magnetic moment of the neutrino. With an energy threshold of about 5 MeV we expect to be sensitive in the range of pv 1 1 X 10-t’ pn for up, v, and 1;. (Fig. 1). The energy resolution should be in the range of u~/E s 5O%o/~z-pG.q m order to analyze the shapes of the energy spectra with sufficient precision and good low
* Corresponding
author.
’ Kartsruhe Rutherford Medium Energy Neutrino experiment. Old-~2/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-9002(95)00108-j
energy threshold behavior. In order to separate v-e scattering from other reactions, the angular resolution should be in the range of 20” (Fig. 2). The high statistics of the t60(v,, e-)mF * reaction in a water Cherenkov detector allows an accurate measurement of the differential cross section, important for astrophysics. On the other hand it is possible to detect neutrino oscillations in the disappearance channel by analysing the shape of the energy spectrum and the total event rates, for example v, -+ v, oscillations in the mass range of nzz, = 2-20 eV2. Both the measurement of the cross section of the inverse B-decay and the search for oscillations have strong implications on astrophysics and are important for other water Cherenkov detectors measuring neutrinos from astrophysical sources. The size of the cross sections involved, of the order of lo-“’ cm2, demands a strong suppression of random background from cosmic muons. This is easily facilitated by the choice of a short pulse structure for the v-source, as in the case of ISIS, in combination with the reconstruction
electron recoil energy
[t&V]
Fig. 1. Cross sections for weak neutrino-electron scattering with respect to the energy of the recoiled electron (solid) and additional electromagnetic scattering due to a magnetic moment of 3 X 10-‘” pa (dashed).
X. PARTICLE
IDENTIFICATION
3. ~rmbruster
et al. /Nucl.
Ins@. and Meth. in Phys. Res. A 360 (19951395-399
addition the extremely small duty factor of neutrinos at ISIS of the order of IO-’ for V~ allows effective suppression of cosmic background. This can be compared with the attenuation of the muon intensity by massive rock above underground neutrino detectors as KAMIOKANDE.
3. Detector geometry
Fig. 2. Reconstructed cos 0 spectra for ~~--e, it-e and oh I60 reactions showing the clearly forward-peaked neutrino-electron scattering and a “background” linear in cos 0 and. enhanced in the backward direction due to r60. With the shown linear fit for cos 8 < 0.5 it is easily possible to separate the reactions.
of the stopping point of cosmic muons in the detector. For this purpose the position resolution should be less than 15 cm forcing a timing resolution of about 1 ns.
2. The neutrino source ISIS The pulsed spallation-neutron facility ISIS at Rutherford Appleton Laboratory is based on an 800 MeV rapidcycling synchrotron with 200 pA average beam current. The protons are stopped in a tantalum/uranium heavy water target and produce 0.0456 n+/p decays at rest: 7rC’
Neutron beam lines of solid state physics experiments limitate space available at the ISIS site at RAL. Thus the minimum distance of the proposed water Cherenkov detector (KARMEN2) from the target is about 15 m. To achieve additional shielding by the surrounding material (mainly chalk) it is proposed to go 14 m beneath ground level. The water tank would be 15.8 m long, 7 m wide and 12 m deep and sited just below the position of the existing JSARMEN detector. KARMEN is shielded by 6000 t of steel against cosmic radiation and beam correlated background. This steel can be used to shield the new detector with 3 m of iron on top and 2.5 m of iron towards the target station. To obtain a sufficient energy resolution one needs a light sensitive coverage of at least 20%. That can be achieved, for example, by an equidistant arrangement of 3100, 10 in. photomultipliers (PMTs). The flux averaged distance to the target is 24.8 m. Assuming a beam current of 3000 C/yr and calculating the flux averaged total cross sections for neutrino-electron scattering and inverse p-decay on 160, yields to 224 VW-e, 1476 v,-e, 245 CCL-eand 4446 ve I60 reconstructed events per year.
p‘++ t$& fT, = 26 ns),
p,++e++u,+pfi
(7@=2.2
(1)
ps).
The time structure of the proton beam (two 100 ns bunches 330 ns apart, recurring at 50 Hz) gives a prompt burst of monoenergetic u@ (.!& = 29.8 MeV) within the first 0.5 p,s after beam-on-target, followed by equal fluxes of v, and pp with continuous energies up to 52.8 MeV (Fig. 3) from muon decay at rest. Neutrinos from pion and muon decay can thus be seperated by time measurement. In
0
10
20
30
40
60
neutrinoenergy [MeV] Fig. 3. Energy spectra of the three neutrino types IJ~, ve and ?n emitted by ISIS.
4. Simulation of the detector response Extensive Monte Carlo calculations have been made to obtain the detector response. Electrons with energy, position, angular and time distribution according to the theoretical cross sections and the properties of the neutrino source are started inside the water volume of the detector and tracked with the help of GEANT 3.15 including effects like bremsstrahlung, Compton and Molibre scattering, etc. The space time points of all produced charged particles are recorded and evaluated in our own program simulating the Cherenkov effect. The number dN of Cherenkov photons in a wavelength interval dh emitted on a track interval dx is calculated according to
where n is the refraction index of water and p the actual velocity of the particle. All Cherenkov photons are emitted equally distributed between two trackpoints with an angle of cos @= l/n/3 relative to the particle track. They are transported to the detector walls, applying losses (19%) for the spectral attenuation length of water
B. Armbruster et al. /Nucl. Instr. and Me& in fhys. Res. A 360 (199.5)395-399
397
niques. The energy of an event is reconstructed from the sum of pulse heights corrected by a position- and direction-dependent factor due to the attenuation in water. Comparison with the start values of events allows one to obtain the detector resolutions presented in Table 1. These resolution figures are sufficient for the mentioned goals of the experiment, due to the chosen time resolution of the PMTs and electronics. The very good properties of the Burle C83061E PMT have been taken just as a reference implying no prejudice for the choice for the actual detector.
5. Energy resolution
Fig. 4. Simulation of a ve-e reaction. Phototube hits are connected with the start position of the electron by solid lines. The three pe~endicu~ar lines along the ~ordinate axes mark the reconstructed position of the event.
(reaching up to 40 m in ultrapure water at 400 nm). Those photons hitting an active photocathode of a PMT, and not being reflected by the PMT glass, are converted to photoelectrons with a probability pro~~ional to the spectral quantum efficiency of the PMT (Fig. 4). The transit time spread, the single photoelectron resolution, and the integration time contribute to the simulated shape of each PMT pulse. A simulated leading edge discriminator and a TDC with a resolution of 1 ns deliver &he hit time of the PMT after walk-correction using the height of the pulse. A trigger condition of at least 10 active PMTs in a 50 ns interval is provided to identify real events among simulated dark noise of the PMTs. The walk-corrected times of triggered PMTs are used to reconstruct position, time and direction of the event applying least-square techTable 1 Comparison of the parameters of ~M~N2 Parameter
There are several effects contributing to the relatively poor energy resolution compared to scintillation counters: first of all the number of collected photons is low. An electron with an energy of 20 MeV emits about 370 photons per cm track length in the wavelength range where water has a suitable attenuation length. Only about 2% of these photons are converted to photoelectrons in KARMENZ But there are other principal effects, which have to be taken into account (see Fig. 5). The photon production rate is not proportions to the energy loss as in scintillators but to the track length of the particles and depends on their actual velocity (Eq. (2)). Bremsstrahlung and multiple scattering lose varying fractions of the primary energy below the Cherenkov threshold /3 2 l/n. The resulting loss-fluctuations give rise to a relative energy resolution of about 5% and are inherent to a water Cherenkov detector. Another effect arises from the incomplete coverage and/or granularity of the detector walls. Events near to the PMTs cause a large fluctuation of the detected photons. The collection efficiency varies strongly with the direction of the Cherenkov cone. This adds another 2% to AE/E for KARMEN2.
with other Cherenkov detectors
Hz0 KARMEN 2
Super-~IO~DE
an, Granularity
1.3 kt 21% 10 in. C83061E Burle 1 ns 4 PM/m”
a&E (I 50 MeV) o;& = 30 MeV) q,(E, = 30 MeV) q(E, = 30 MeV) o0 (Throughgoing p’s) Ethreshold
5.2% f 47%/G 0.6 ns 13 cm 20” 2.6” 5.5 MeV
32 kt 40% 20 in. R3600 Hamamatsu 2.6 ns 2 PM/m” 44.3%/G 3 ns at 10 MeV 50 cm at 10 MeV 27” at 10 MeV 1” 4-5 MeV
Fiducial volume Photosensitive coverage Type of PMT
[_5f
Mineral oil LSND [3] 0.2 kt 26% 8 in. R4558 Hamamatsu 1.6 ns 6 PM/m2 42%,$&E0.8 ns 14 cm 12” l-6 MeV
X. PARTICLE I~~NT~FI~ATION
398
B. Armbruster
et al. /Nucl.
Instr. and Meth. in Phys. Res. A 360 (1995) 395-399
Fig. 5. various contributions to the relative energy resoltion. A square root term together with a constant offset describes the
behaviour.
The Cherenkov part cannot be diminished by higher coverage with more PMTs. Therein the energy resolution may be parametrized with a term proportional to the statistical error and a constant offset, as shown in Fig. 5. LSND [3] is a Cherenkov detector based on mineral oil with additional scintillator. The ratio between Cherenkov and scintillator photons is 1: 3, thus the energy resolution is dominated by the photon production of the scintillator and no constant offset is needed to describe the energy resolution (Table 1).
volume around the stopping position and to suppress the decay Michel electrons occurring with a lifetime of 2.0 ps. About 18% of all stopped pm are captured by IhO. 2% of the p*- produce bound states of 16N [4], an important source of background due to its subsequent B-decay with a lifetime of 7.13 s and endpoint energies up to 10.42 MeV (Fig. 6). Combining the capture probability of p-, the stopping rate, and the p+/p. ratio, a production rate of 29 Hz is estimated. The histogram in Fig. 6 shows the reconstructed, visible energy of these events, including the photons produced by converted y’s emitted in 66% of the B-decays. The trigger rate is about 12 Hz yielding up to 9 x lo4 I6N events in the neutrino time window per year. Background subtraction will yield unacceptable fluctuations for the neutrino signal below 8 MeV. Furthermore, identification of 16N-decays by detection of the y quanta, preferentially opposite to the electron, may reduce the background only by factor 2 or so. Applying a dead time of several seconds after each stopped muon where no Michel electron is detected would yield a dead time of approximately 60% in the energy region below 8 MeV. This background caused by muon capture seems to limit all lower energy water Cherenkov experiments near and above sea level to an energy range above 8 MeV.
7. Conclusion 6. Muon capture on oxygen A kiloton detector at sea level has to take care of cosmogenic background. It is planned to shield the detector with 3 m of massive iron against cosmic particles eliminating the hadronic component, but about 60% of the cosmic muons will pass the shielding. We expect 3.3 kHz of stopped and 5.9 kHz of throughgoing muons. Identifying stopped muons with a hermetic anti-counter and reconstructing the stopping position, allows one to place a dead
visible
energy
[MeV]
Fig. 6. Solid line: energy spectrum of the 16N decay. The main component (66% abundance) with an endpoint energy of 4.29 MeV is accompanied by the marked y. Histogram: visible reconstructed energy of 16N events. The energy of the electrons and the y’s add up to an amount well above the threshold of the detector.
and outlook
At a beam dump neutrino source a large imaging water Cherenkov detector seems to be ideally suited to study low energy neutrino-electron scattering down to 8 MeV. The forward peaked cross section of this reaction, and the ability of a Cherenkov detector to reconstruct the direction of the scattered electron, lead to a clear signature, so that more isotropic reactions may be subtracted, leading to a high statistics and high quality signal. Compared to Super-KAMIOKANDE, KARMEN2 would reach similar (energy) or even better (time, position) resolutions with much less effort in number and size of PMTs. The energy resolution of LSND is comparable to KARMEN2 although the detector is rather small and surface effects should strongly contribute, but the use of mineral oil in connection with a small amount scintillator compensates these problems. In 1994 we started to build a prototype with 27 t of water (size 3 X 3 X 3 m3> to measure signatures of cosmic ray induced reactions. First tests of 8 in. EM1 9353 PMTs revealing a transit time spread of about 1 ns, good single photoelectron resolution (37%) together with a clearly visible single photoelectron peak (peak to valley 2.6) and a high quantum efficiency (30% at 350 nm) of the photocathode, favour the use of 96 of these PMTs for a prototype detector: 4 X 4 equally arranged on each side of a cubic tank. Main goals of the prototype are the calibration of the detector Monte Carlo in order to prepare a detector proposal, the measurement of real cosmogenic background
B. Armbruster
at sea level, and the development electronics and data acquisition.
et al. /Nucl.
Instr. and Meth. in Phys. Res. A 360 (1995) 395-399
of an optimal design of
399
[2] B. Armbruster et al., Prog. Part. Nucl. Phys. 32 (1994) 397.
[3] X-Q. Lu et al., Los Alamos, LA-11842-P. [4] J.P. Deutsch et al., Nuovo Cimento 52B (1967) 557. [5] Y. Totsuka, ICRR-Report 227-90-20 (1990).
References [l] B. Zeitnitz et al. (KARMEN Collaboration), Prog. Part. Nucl. Phys. 32 (1994) 351.
X. PARTICLE IDENTIFICATION
Nuclear Instruments and Methods in Physics Research A 360 (199.5)395-399
NUCLEAR INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH
__ LiiB
SectIonA
ELSEWIER
Study of a water Cherenkov detector for low energy v-e scattering B. Armbruster,
G. Drexlin, V. Eberhard, H. Gemmeke
*, B. Zeitnitz
Forschungszentrum Karlsruhe and Universiq Karlsruhe, Karlsruhe, Germany
Abstract For low energy tests of the standard model by v-e scattering and the determination of v--‘~O cross sections a high resolution 1.3 kt Hz0 Cherenkov detector is under evaluation. The results of extensive Monte Carlo simulations are presented. The goal is a high resolution Cherenkov detector with time resolution of 0.6 ns and energy resolution of 17% at E, = 15 MeV.
1. ~nt~duction We propose to use a water Cherenkov detector to proceed in the sequence of high precision tests of the weak interaction with KARMEN ’ [l]. A water Cherenkov detector is an ideal instrument to study the strongly forward peaked neutrino-electron scattering in the presence of more isotropic distributed reactions, as inverse B-decay on 160 and muon induced background [2]. In addition such a detector is not very sensitive to the low energy neutron background unavoidably produced in a massive beam dump u-source. Furthermore, a water Cherenkov detector has the advantage to determine the hitherto unmeasured v~-‘~O cross section which is important for neutrino astrophysics, whereas a mineral oil based detector as LSND would remeasure the v-t? cross section well known from KARMEN. Neutrino reactions in water cover a wide range of physical topics. Neutrino-electron scattering of the type u,--e allows us to measure precisely the interference of neutral- and charged-current reactions. The measurement of the differential cross section, especially of the VI,--e reaction, allows one to extract the ratio of the left handed and right handed coupling constants, &gf, in a flux independent manner from the shape of the spectrum. The cross section of w-e scattering is strongly enhanced in the low energy region by a hypothetical magnetic moment of the neutrino. With an energy threshold of about 5 MeV we expect to be sensitive in the range of pv 1 1 X 10-t’ pn for up, v, and 1;. (Fig. 1). The energy resolution should be in the range of u~/E s 5O%o/~z-pG.q m order to analyze the shapes of the energy spectra with sufficient precision and good low
* Corresponding
author.
’ Kartsruhe Rutherford Medium Energy Neutrino experiment. Old-~2/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-9002(95)00108-j
energy threshold behavior. In order to separate v-e scattering from other reactions, the angular resolution should be in the range of 20” (Fig. 2). The high statistics of the t60(v,, e-)mF * reaction in a water Cherenkov detector allows an accurate measurement of the differential cross section, important for astrophysics. On the other hand it is possible to detect neutrino oscillations in the disappearance channel by analysing the shape of the energy spectrum and the total event rates, for example v, -+ v, oscillations in the mass range of nzz, = 2-20 eV2. Both the measurement of the cross section of the inverse B-decay and the search for oscillations have strong implications on astrophysics and are important for other water Cherenkov detectors measuring neutrinos from astrophysical sources. The size of the cross sections involved, of the order of lo-“’ cm2, demands a strong suppression of random background from cosmic muons. This is easily facilitated by the choice of a short pulse structure for the v-source, as in the case of ISIS, in combination with the reconstruction
electron recoil energy
[t&V]
Fig. 1. Cross sections for weak neutrino-electron scattering with respect to the energy of the recoiled electron (solid) and additional electromagnetic scattering due to a magnetic moment of 3 X 10-‘” pa (dashed).
X. PARTICLE
IDENTIFICATION
3. ~rmbruster
et al. /Nucl.
Ins@. and Meth. in Phys. Res. A 360 (19951395-399
addition the extremely small duty factor of neutrinos at ISIS of the order of IO-’ for V~ allows effective suppression of cosmic background. This can be compared with the attenuation of the muon intensity by massive rock above underground neutrino detectors as KAMIOKANDE.
3. Detector geometry
Fig. 2. Reconstructed cos 0 spectra for ~~--e, it-e and oh I60 reactions showing the clearly forward-peaked neutrino-electron scattering and a “background” linear in cos 0 and. enhanced in the backward direction due to r60. With the shown linear fit for cos 8 < 0.5 it is easily possible to separate the reactions.
of the stopping point of cosmic muons in the detector. For this purpose the position resolution should be less than 15 cm forcing a timing resolution of about 1 ns.
2. The neutrino source ISIS The pulsed spallation-neutron facility ISIS at Rutherford Appleton Laboratory is based on an 800 MeV rapidcycling synchrotron with 200 pA average beam current. The protons are stopped in a tantalum/uranium heavy water target and produce 0.0456 n+/p decays at rest: 7rC’
Neutron beam lines of solid state physics experiments limitate space available at the ISIS site at RAL. Thus the minimum distance of the proposed water Cherenkov detector (KARMEN2) from the target is about 15 m. To achieve additional shielding by the surrounding material (mainly chalk) it is proposed to go 14 m beneath ground level. The water tank would be 15.8 m long, 7 m wide and 12 m deep and sited just below the position of the existing JSARMEN detector. KARMEN is shielded by 6000 t of steel against cosmic radiation and beam correlated background. This steel can be used to shield the new detector with 3 m of iron on top and 2.5 m of iron towards the target station. To obtain a sufficient energy resolution one needs a light sensitive coverage of at least 20%. That can be achieved, for example, by an equidistant arrangement of 3100, 10 in. photomultipliers (PMTs). The flux averaged distance to the target is 24.8 m. Assuming a beam current of 3000 C/yr and calculating the flux averaged total cross sections for neutrino-electron scattering and inverse p-decay on 160, yields to 224 VW-e, 1476 v,-e, 245 CCL-eand 4446 ve I60 reconstructed events per year.
p‘++ t$& fT, = 26 ns),
p,++e++u,+pfi
(7@=2.2
(1)
ps).
The time structure of the proton beam (two 100 ns bunches 330 ns apart, recurring at 50 Hz) gives a prompt burst of monoenergetic u@ (.!& = 29.8 MeV) within the first 0.5 p,s after beam-on-target, followed by equal fluxes of v, and pp with continuous energies up to 52.8 MeV (Fig. 3) from muon decay at rest. Neutrinos from pion and muon decay can thus be seperated by time measurement. In
0
10
20
30
40
60
neutrinoenergy [MeV] Fig. 3. Energy spectra of the three neutrino types IJ~, ve and ?n emitted by ISIS.
4. Simulation of the detector response Extensive Monte Carlo calculations have been made to obtain the detector response. Electrons with energy, position, angular and time distribution according to the theoretical cross sections and the properties of the neutrino source are started inside the water volume of the detector and tracked with the help of GEANT 3.15 including effects like bremsstrahlung, Compton and Molibre scattering, etc. The space time points of all produced charged particles are recorded and evaluated in our own program simulating the Cherenkov effect. The number dN of Cherenkov photons in a wavelength interval dh emitted on a track interval dx is calculated according to
where n is the refraction index of water and p the actual velocity of the particle. All Cherenkov photons are emitted equally distributed between two trackpoints with an angle of cos @= l/n/3 relative to the particle track. They are transported to the detector walls, applying losses (19%) for the spectral attenuation length of water
B. Armbruster et al. /Nucl. Instr. and Me& in fhys. Res. A 360 (199.5)395-399
397
niques. The energy of an event is reconstructed from the sum of pulse heights corrected by a position- and direction-dependent factor due to the attenuation in water. Comparison with the start values of events allows one to obtain the detector resolutions presented in Table 1. These resolution figures are sufficient for the mentioned goals of the experiment, due to the chosen time resolution of the PMTs and electronics. The very good properties of the Burle C83061E PMT have been taken just as a reference implying no prejudice for the choice for the actual detector.
5. Energy resolution
Fig. 4. Simulation of a ve-e reaction. Phototube hits are connected with the start position of the electron by solid lines. The three pe~endicu~ar lines along the ~ordinate axes mark the reconstructed position of the event.
(reaching up to 40 m in ultrapure water at 400 nm). Those photons hitting an active photocathode of a PMT, and not being reflected by the PMT glass, are converted to photoelectrons with a probability pro~~ional to the spectral quantum efficiency of the PMT (Fig. 4). The transit time spread, the single photoelectron resolution, and the integration time contribute to the simulated shape of each PMT pulse. A simulated leading edge discriminator and a TDC with a resolution of 1 ns deliver &he hit time of the PMT after walk-correction using the height of the pulse. A trigger condition of at least 10 active PMTs in a 50 ns interval is provided to identify real events among simulated dark noise of the PMTs. The walk-corrected times of triggered PMTs are used to reconstruct position, time and direction of the event applying least-square techTable 1 Comparison of the parameters of ~M~N2 Parameter
There are several effects contributing to the relatively poor energy resolution compared to scintillation counters: first of all the number of collected photons is low. An electron with an energy of 20 MeV emits about 370 photons per cm track length in the wavelength range where water has a suitable attenuation length. Only about 2% of these photons are converted to photoelectrons in KARMENZ But there are other principal effects, which have to be taken into account (see Fig. 5). The photon production rate is not proportions to the energy loss as in scintillators but to the track length of the particles and depends on their actual velocity (Eq. (2)). Bremsstrahlung and multiple scattering lose varying fractions of the primary energy below the Cherenkov threshold /3 2 l/n. The resulting loss-fluctuations give rise to a relative energy resolution of about 5% and are inherent to a water Cherenkov detector. Another effect arises from the incomplete coverage and/or granularity of the detector walls. Events near to the PMTs cause a large fluctuation of the detected photons. The collection efficiency varies strongly with the direction of the Cherenkov cone. This adds another 2% to AE/E for KARMEN2.
with other Cherenkov detectors
Hz0 KARMEN 2
Super-~IO~DE
an, Granularity
1.3 kt 21% 10 in. C83061E Burle 1 ns 4 PM/m”
a&E (I 50 MeV) o;& = 30 MeV) q,(E, = 30 MeV) q(E, = 30 MeV) o0 (Throughgoing p’s) Ethreshold
5.2% f 47%/G 0.6 ns 13 cm 20” 2.6” 5.5 MeV
32 kt 40% 20 in. R3600 Hamamatsu 2.6 ns 2 PM/m” 44.3%/G 3 ns at 10 MeV 50 cm at 10 MeV 27” at 10 MeV 1” 4-5 MeV
Fiducial volume Photosensitive coverage Type of PMT
[_5f
Mineral oil LSND [3] 0.2 kt 26% 8 in. R4558 Hamamatsu 1.6 ns 6 PM/m2 42%,$&E0.8 ns 14 cm 12” l-6 MeV
X. PARTICLE I~~NT~FI~ATION
398
B. Armbruster
et al. /Nucl.
Instr. and Meth. in Phys. Res. A 360 (1995) 395-399
Fig. 5. various contributions to the relative energy resoltion. A square root term together with a constant offset describes the
behaviour.
The Cherenkov part cannot be diminished by higher coverage with more PMTs. Therein the energy resolution may be parametrized with a term proportional to the statistical error and a constant offset, as shown in Fig. 5. LSND [3] is a Cherenkov detector based on mineral oil with additional scintillator. The ratio between Cherenkov and scintillator photons is 1: 3, thus the energy resolution is dominated by the photon production of the scintillator and no constant offset is needed to describe the energy resolution (Table 1).
volume around the stopping position and to suppress the decay Michel electrons occurring with a lifetime of 2.0 ps. About 18% of all stopped pm are captured by IhO. 2% of the p*- produce bound states of 16N [4], an important source of background due to its subsequent B-decay with a lifetime of 7.13 s and endpoint energies up to 10.42 MeV (Fig. 6). Combining the capture probability of p-, the stopping rate, and the p+/p. ratio, a production rate of 29 Hz is estimated. The histogram in Fig. 6 shows the reconstructed, visible energy of these events, including the photons produced by converted y’s emitted in 66% of the B-decays. The trigger rate is about 12 Hz yielding up to 9 x lo4 I6N events in the neutrino time window per year. Background subtraction will yield unacceptable fluctuations for the neutrino signal below 8 MeV. Furthermore, identification of 16N-decays by detection of the y quanta, preferentially opposite to the electron, may reduce the background only by factor 2 or so. Applying a dead time of several seconds after each stopped muon where no Michel electron is detected would yield a dead time of approximately 60% in the energy region below 8 MeV. This background caused by muon capture seems to limit all lower energy water Cherenkov experiments near and above sea level to an energy range above 8 MeV.
7. Conclusion 6. Muon capture on oxygen A kiloton detector at sea level has to take care of cosmogenic background. It is planned to shield the detector with 3 m of massive iron against cosmic particles eliminating the hadronic component, but about 60% of the cosmic muons will pass the shielding. We expect 3.3 kHz of stopped and 5.9 kHz of throughgoing muons. Identifying stopped muons with a hermetic anti-counter and reconstructing the stopping position, allows one to place a dead
visible
energy
[MeV]
Fig. 6. Solid line: energy spectrum of the 16N decay. The main component (66% abundance) with an endpoint energy of 4.29 MeV is accompanied by the marked y. Histogram: visible reconstructed energy of 16N events. The energy of the electrons and the y’s add up to an amount well above the threshold of the detector.
and outlook
At a beam dump neutrino source a large imaging water Cherenkov detector seems to be ideally suited to study low energy neutrino-electron scattering down to 8 MeV. The forward peaked cross section of this reaction, and the ability of a Cherenkov detector to reconstruct the direction of the scattered electron, lead to a clear signature, so that more isotropic reactions may be subtracted, leading to a high statistics and high quality signal. Compared to Super-KAMIOKANDE, KARMEN2 would reach similar (energy) or even better (time, position) resolutions with much less effort in number and size of PMTs. The energy resolution of LSND is comparable to KARMEN2 although the detector is rather small and surface effects should strongly contribute, but the use of mineral oil in connection with a small amount scintillator compensates these problems. In 1994 we started to build a prototype with 27 t of water (size 3 X 3 X 3 m3> to measure signatures of cosmic ray induced reactions. First tests of 8 in. EM1 9353 PMTs revealing a transit time spread of about 1 ns, good single photoelectron resolution (37%) together with a clearly visible single photoelectron peak (peak to valley 2.6) and a high quantum efficiency (30% at 350 nm) of the photocathode, favour the use of 96 of these PMTs for a prototype detector: 4 X 4 equally arranged on each side of a cubic tank. Main goals of the prototype are the calibration of the detector Monte Carlo in order to prepare a detector proposal, the measurement of real cosmogenic background
B. Armbruster
at sea level, and the development electronics and data acquisition.
et al. /Nucl.
Instr. and Meth. in Phys. Res. A 360 (1995) 395-399
of an optimal design of
399
[2] B. Armbruster et al., Prog. Part. Nucl. Phys. 32 (1994) 397.
[3] X-Q. Lu et al., Los Alamos, LA-11842-P. [4] J.P. Deutsch et al., Nuovo Cimento 52B (1967) 557. [5] Y. Totsuka, ICRR-Report 227-90-20 (1990).
References [l] B. Zeitnitz et al. (KARMEN Collaboration), Prog. Part. Nucl. Phys. 32 (1994) 351.
X. PARTICLE IDENTIFICATION