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Upgrade of LXe gamma-ray detector in MEG experiment
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Nuclear Physics B (Proc. Suppl.) 253–255 (2014) 214–215 www.elsevier.com/locate/npbps
Upgrade of LXe gamma-ray detector in MEG experiment Daisuke Kaneko, on behalf of MEG collaboration ICEPP, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Abstract We are searching for the μ → e+γ in the MEG experiment. In order to improve the search sensitivity down to about 5 × 10−14 , which is one order higher than the sensitivity goal of the current stage of the experiment. We are planning a major upgrade of the experiment including the upgrade of the liquid xenon (LXe) γ-ray detector. The main item of the upgrade of the xenon detector is a replacement of the current PMTs located on the γ-ray incident face with smaller photosensors such as MPPC. The energy and position resolutions are expected to be significantly improved, especially for the event where γ-ray converts at a shallow point in the LXe. Because the MPPC operational in liquid xenon is not yet commercially available, we are developing special MPPC in collaboration with Hamamatsu Photonics. The detection efficiency for LXe scintillation light is measured with prototype sensors and found to be already at the level necessary for the upgrade of the experiment. Keywords: LFV, MEG, liquid xenon detector, MPPC
1. Status and Prospects of MEG experiment We are searching for the charged lepton flavour violating decay μ → e + γ at Paul Scherrer Institute in Switzerland from year 2008. A result of combined data of year 2009 and 2010 has been published where the upper limit for the branching ratio is set to be 2.4 × 10−12 at 90% CL [1]. This is the most stringent experimental upper limit ever on this decay mode. We are aiming at reaching the sensitivity goal of the first phase of the MEG experiment of 6 × 10−13 in year 2013. In order to achieve one order higher sensitivity, a major upgrade is being planned where the upgrade of the liquid xenon detector is included.
it is known that events where γ-ray converts at shallow position (near to a face of γ incidence) have worse energy resolution. It is because there is a fluctuation of geometrical efficiency of scintillation photon collection which depends on relative position between PMTs array and γ conversion. Position resolution is also limited by the size of the sensors. In order to attain higher resolution, it is planned to replace the current 2-inch PMT on the γ-ray incident face with smaller photosensor such as MPPC (the product name of silicon photomultiplier from Hamamatsu Photonics).
2. Concept of Liquid Xenon Detector Upgrade The role of the liquid xenon detector is to detect γray from muon decay. The detector is filled with liquid xenon, which is surrounded by 846 PMTs to collect scintillation photons. Good resolutions of energy, position and timing are important in order to reject backgrounds and thus to achieve high sensitivity. However, http://dx.doi.org/10.1016/j.nuclphysbps.2014.09.055 0920-5632/© 2014 Elsevier B.V. All rights reserved.
Figure 1: Modification of PMT layout shown in cross-sectional view of LXe detector.
A modification of PMT layout on lateral faces is also
D. Kaneko / Nuclear Physics B (Proc. Suppl.) 253–255 (2014) 214–215
215
to our xenon-detector, there are some issues to be resolved. One of them is the low sensitivity of MPPC commercially available to LXe scintillation light. Since its wavelength lies around 175nm, most scintillation photons are absorbed at non-sensitive region. To improve photon detection efficiency (PDE), various samples are made. 5. Results from Recent Test Figure 2: Energy spectra of signal γ events: left and right side Figure 3: Position resolution vs show current and updated situ- depth of γ-ray conversion ation respectively
planned as shown in Figure 1. By widening the γ incidence face, energy leakage of event near the lateral face will be reduced. The layout of PMT will be modified aiming for more uniform response.
Figure 5 shows the result of the PDE measurement. PDE is calculated as the ratio of the observed number of photoelectrons for α from 241 Am to the expectation. In this plot, the effects of the optical crosstalk and the afterpulsing, which are separately measured, are subtracted.
3. Expected Performance The performance of the upgraded detector is evaluated by Monte Carlo simulation. Figure 2 shows a comparison of the energy resolution between the current and upgraded detectors. The improvement of the energy resolution is remarkable especially in shallow part. The resolution is improved from 2.1% to 0.5% in σ. Figure 3 shows the position resolution as a function of depth. In the current detector, the position resolution is worse in shallow part due to the limitation by the sensor size. With MPPC, the resolution becomes better in this region. Figure 4 shows an example of the comparison of light distribution, where we can see that the imaging power is clearly improved.
Figure 5: Measured PDE as a function of voltage over breakdown
Currently, the PDE of the best samples is about 10% (shown as ”G” type, in figure 5). This is lower than that of current PMT. However since the sensor coverage area is improved with MPPCs, the expected total number of photoelectorns is almost the same as that in the current detector. 6. Conclusion
Thanks to the much smaller thickness of MPPC, the detection efficiency for signal γ will improve by about 10% with MPPC.
A UV-sensitive MPPC is under development for the upgrade of the MEG LXe detector. We already obtained a reasonable PDE of about 10% for the best sample. Next we will test the MPPC samples of realistic size(12 × 12mm2 ), and finalize the parameters of the MPPC to be used in the experiment. A test with a prototype LXe detector with several hundred UV-sensitive MPPCs is planned in year 2013 to demonstrate the performance of the concept of the upgrade.
4. Development of UV-sensitive MPPC
References
We are developing a new type of MPPCs in collaboration with Hamamatsu Photonics. To apply MPPCs
[1] J. Adam et al., New limit on the lepton-flavor-violating decay μ+ → e+ γ, Phys. Rev. Lett. 107 (2011) 171801.
Figure 4: Comparison of photon distribution with different sensor types on γ entrance face
Nuclear Physics B (Proc. Suppl.) 253–255 (2014) 214–215 www.elsevier.com/locate/npbps
Upgrade of LXe gamma-ray detector in MEG experiment Daisuke Kaneko, on behalf of MEG collaboration ICEPP, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Abstract We are searching for the μ → e+γ in the MEG experiment. In order to improve the search sensitivity down to about 5 × 10−14 , which is one order higher than the sensitivity goal of the current stage of the experiment. We are planning a major upgrade of the experiment including the upgrade of the liquid xenon (LXe) γ-ray detector. The main item of the upgrade of the xenon detector is a replacement of the current PMTs located on the γ-ray incident face with smaller photosensors such as MPPC. The energy and position resolutions are expected to be significantly improved, especially for the event where γ-ray converts at a shallow point in the LXe. Because the MPPC operational in liquid xenon is not yet commercially available, we are developing special MPPC in collaboration with Hamamatsu Photonics. The detection efficiency for LXe scintillation light is measured with prototype sensors and found to be already at the level necessary for the upgrade of the experiment. Keywords: LFV, MEG, liquid xenon detector, MPPC
1. Status and Prospects of MEG experiment We are searching for the charged lepton flavour violating decay μ → e + γ at Paul Scherrer Institute in Switzerland from year 2008. A result of combined data of year 2009 and 2010 has been published where the upper limit for the branching ratio is set to be 2.4 × 10−12 at 90% CL [1]. This is the most stringent experimental upper limit ever on this decay mode. We are aiming at reaching the sensitivity goal of the first phase of the MEG experiment of 6 × 10−13 in year 2013. In order to achieve one order higher sensitivity, a major upgrade is being planned where the upgrade of the liquid xenon detector is included.
it is known that events where γ-ray converts at shallow position (near to a face of γ incidence) have worse energy resolution. It is because there is a fluctuation of geometrical efficiency of scintillation photon collection which depends on relative position between PMTs array and γ conversion. Position resolution is also limited by the size of the sensors. In order to attain higher resolution, it is planned to replace the current 2-inch PMT on the γ-ray incident face with smaller photosensor such as MPPC (the product name of silicon photomultiplier from Hamamatsu Photonics).
2. Concept of Liquid Xenon Detector Upgrade The role of the liquid xenon detector is to detect γray from muon decay. The detector is filled with liquid xenon, which is surrounded by 846 PMTs to collect scintillation photons. Good resolutions of energy, position and timing are important in order to reject backgrounds and thus to achieve high sensitivity. However, http://dx.doi.org/10.1016/j.nuclphysbps.2014.09.055 0920-5632/© 2014 Elsevier B.V. All rights reserved.
Figure 1: Modification of PMT layout shown in cross-sectional view of LXe detector.
A modification of PMT layout on lateral faces is also
D. Kaneko / Nuclear Physics B (Proc. Suppl.) 253–255 (2014) 214–215
215
to our xenon-detector, there are some issues to be resolved. One of them is the low sensitivity of MPPC commercially available to LXe scintillation light. Since its wavelength lies around 175nm, most scintillation photons are absorbed at non-sensitive region. To improve photon detection efficiency (PDE), various samples are made. 5. Results from Recent Test Figure 2: Energy spectra of signal γ events: left and right side Figure 3: Position resolution vs show current and updated situ- depth of γ-ray conversion ation respectively
planned as shown in Figure 1. By widening the γ incidence face, energy leakage of event near the lateral face will be reduced. The layout of PMT will be modified aiming for more uniform response.
Figure 5 shows the result of the PDE measurement. PDE is calculated as the ratio of the observed number of photoelectrons for α from 241 Am to the expectation. In this plot, the effects of the optical crosstalk and the afterpulsing, which are separately measured, are subtracted.
3. Expected Performance The performance of the upgraded detector is evaluated by Monte Carlo simulation. Figure 2 shows a comparison of the energy resolution between the current and upgraded detectors. The improvement of the energy resolution is remarkable especially in shallow part. The resolution is improved from 2.1% to 0.5% in σ. Figure 3 shows the position resolution as a function of depth. In the current detector, the position resolution is worse in shallow part due to the limitation by the sensor size. With MPPC, the resolution becomes better in this region. Figure 4 shows an example of the comparison of light distribution, where we can see that the imaging power is clearly improved.
Figure 5: Measured PDE as a function of voltage over breakdown
Currently, the PDE of the best samples is about 10% (shown as ”G” type, in figure 5). This is lower than that of current PMT. However since the sensor coverage area is improved with MPPCs, the expected total number of photoelectorns is almost the same as that in the current detector. 6. Conclusion
Thanks to the much smaller thickness of MPPC, the detection efficiency for signal γ will improve by about 10% with MPPC.
A UV-sensitive MPPC is under development for the upgrade of the MEG LXe detector. We already obtained a reasonable PDE of about 10% for the best sample. Next we will test the MPPC samples of realistic size(12 × 12mm2 ), and finalize the parameters of the MPPC to be used in the experiment. A test with a prototype LXe detector with several hundred UV-sensitive MPPCs is planned in year 2013 to demonstrate the performance of the concept of the upgrade.
4. Development of UV-sensitive MPPC
References
We are developing a new type of MPPCs in collaboration with Hamamatsu Photonics. To apply MPPCs
[1] J. Adam et al., New limit on the lepton-flavor-violating decay μ+ → e+ γ, Phys. Rev. Lett. 107 (2011) 171801.
Figure 4: Comparison of photon distribution with different sensor types on γ entrance face