1. R&D works on Liquid Xenon Photon Detector for μ  e γ experiment at PSI Satoshi Mihara ICEPP, Univ. of Tokyo Outline Introduction Prototype R&D works Summary

2. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 2 Introduction, μ  e γ at PSI Detect e + and γ, “back to back” and “in time” 100% duty factor continuous beam of ～ 10 8 μ /sec Liquid Xe photon detector Solenoidal magnetic spectrometer with a graded magnetic field aims to achieve the sensitivity down to Br( μ  e γ ) ～ 10 -14 http://meg.psi.ch

3. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 3 Introduction,cont’d μ beam stopped on the target 10 8 /sec  E e =52.8MeV E γ =52.8MeV Back to back, in time Sensitivity  N m =1x10 8 /sec, 2.2x10 7 sec running Ω /4 π =0.09,ε e =0.95, ε γ =0.7, and ε sel =0.8  Single Event sensitivity : 0.94x10 -14 Main background sources  Radiative μ + decay  Accidental overlap NOT back to back, NOT in time  Reduced down to 10 -15 level  eνν γ e γ ν ν e ν ν ? γ μ  e νν +”γ” e γ μ

4. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 4 Requirement on the Photon Detector Good Energy Resolution Good Position Resolution Good time Resolution

5. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 5 Liquid Xe Photon Detector Detect scintillation light(λ=175nm) from Liq. Xe(-100 ℃ ) Fast response, Good Energy, and Position resolutions  W ph = 24 eV (c.f. W ph (NaI) = 17eV)  τ fast =45nsec  Mini-Kamiokande type ! NaI: too slow CsI, BGO: poor resolution at 52.8MeV Inhomoginity to cover large area 800 liter liquid Xe 800 PMTs inside liquid(HAMAMATSU R6041Q)

6. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 6 Small Prototype Large Prototype Final Detector Strategy for the Detector Construction PMT Development Refrigerator Development Purification System Development Attenuation Length Measurement PMT Development Refrigerator Development Purification System Development Attenuation Length Measurement

7. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 7 32 2-inch PMTs surround the active volume of 2.34 liter γ-ray sources of Cr,Cs,Mn, and Y α source for PMT calibration Small Prototype of Liquid Xe Photon Detector Metal channel dynodes Possible to be operated at low -100 o Silica window to transmit UV light Typical gain 10 6, Typical Q.E. 10% Possible to sustain up to 3 atom

8. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 8 Small Prototype Experimental Procedure Xenon liquified with a nitrogen cooling pipe Kept in stable temp.(-100 o ) and pressure (1.2 atom) by controlling the flow of liquid nitrogen PMT operation in liquid xenon has been successful for more than one month

9. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 9 Small Prototype Signal from PMTs Typical PMT output for liq. Xe scintillation light Triggering condition for γ ray events (ex. For g from Mn)  > 220 p.e. in R1  > 50 p.e. in R2  > 10 p.e. in R3  > 100 p.e. in E1  > 20 p.e. in E2 Loose enough to trigger Almost all events where γ interacted well inside the sensitive volume 

10. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 10 Small Prototype Analysis Position of γ interaction : Weighting the position of the PMTs with their individual pulse heights For selecting the fully contained events: Requiring the γ int. position should lie within a central region of 2cmx1cmφ

11. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 11 Small Prototype Result, Energy Resolution Fitting the spectrums with an asymmetric Gaussian function. Results are compared with MC prediction. Simulation of γ int. and energy deposition : EGS4 Simulation of the propagation of scint. Light EGS cut off energy : 1keV Att. Length (absorption):29cm Wph = 24eV 0.7% in σ is expected at 52.8MeV.

12. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 12 PMTs are divided into two groups by the y-z plane.  γ int. positions are calculated in each group and then compared with each other.  Position resolution is estimated as s z1-z2 /√2 Possible to achieve for 52.8MeV γ σ<3mm in position meas. Small Prototype Result, Position Resolution

13. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 13 Small Prototype Result, Time Resolution PMTs are divided again into two groups by the y-z plane. In each group the average of the time measured by TDC is calculated after slewing correction for each PMT. The time resolution is estimated by taking the difference between two groups. Resolution improves as ～ 1/√Npe σ<50psec at 52.8 MeV.

14. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 14 Short Summary on the Small Prototype Energy 0.7% Position <3mm Time <50psec in σ at 52.8MeV Excellent! Go to the next step Large Prototype Go to the next step Large Prototype Extrapolation to Higher Energy

15. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 15 Large Prototype How Large? To study the detector response to higher energy γ rays, large volume is required to fully contain events. Depth from Xe surface(cm) Energy Deposit(arbitrary unit) ～ 40cm x 40cm x 50cm Active Volume ～ 40cm x 40cm x 50cm Active Volume Depth 224 PMTs

16. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 16 Large Prototype Vessel Design Vessel should be large enough to install the detector. Possibility to reuse for future PMT calibration. Thinner entrance window thickness. Thermal insulation. Most of all components used in the final detector should be tested in Large Prototype.  Refrigerator  Feedthrough  PMT holder structure  Surface level meter etc, etc…

17. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 17 Construction of the Large Prototype Vessel

18. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 18 Construction of the Large Prototype Vessel Pressure test up to 0.6MPa Vacuum leak test Cooling test with liquid nitrogen All OK. Ready to fill liquid xenon inside

19. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 19 Assembling Assembling Holder  Front: G10, Plastic  Side, Rear:Aluminum Blue LEDs for PMT gain adjustment α sources for calibration Temperature sensors Surface Level meter

20. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 20 Pulse Tube Refrigerator Conventionally liq. Nitrogen has been used  waste of nitrogen, not suitable for long term physics experiment. Purse tube refrigerator is the best for this purpose since there is no moving part in the low temperature stage. Pressure Oscillation 70W Mechanically moving

21. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 21 Liquefaction Test Pre-cooling before liquefaction with 0.2MPa gas xenon inside. Liquefaction of xenon 100 liter liquid for ~ 2 day (2.0 liter/hour). Recondensation using the refrigerator was successfully done. Several basic measurements done. PMTs operated for 2.5 days stably. Calibration with LED and α source signals done Currently preparing γ beam test in June.

22. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 22 γ beam at TERAS Use inverse-compton scattered γ beam provided at TERAS in ETL AIST in Tsukuba, Japan.  Eγ:10,20,30,40MeV  γ beam intensity: 1kHz (typ.) Analyze the edge of the Compton photon spectrum. Test will start in the middle of June. 40MeV γ Taken after acc. trouble. Better BG condition now! NaI

23. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 23 Plan for Attenuation Length Measurement Step1 PMT1:Absorption + Scattering Length meas. PMT2: used as a reference. Collimators to prevent scinti. light from hitting the wall. Mask in front of PMT1 to define the illuminated area on the photo cathode. Step1 PMT1:Absorption + Scattering Length meas. PMT2: used as a reference. Collimators to prevent scinti. light from hitting the wall. Mask in front of PMT1 to define the illuminated area on the photo cathode. PMT1 PMT2 Liq. Nitrogen X ray Step2 PMT2: Scatt. Length meas. PMT1: reference at a fixed point. Step2 PMT2: Scatt. Length meas. PMT1: reference at a fixed point. dL/L ~ 5 x L(m) [%]

24. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 24 Summary Small Prototype  Small prototype of liquid Xe photon detector was successfully constructed and tested with γ-ray sources of 320keV-1836keV.  Scintillation light from xenon is directly observed with PMTs located inside the liquid.  Excellent Energy, Position, Time resolutions. Large Prototype  100 liter liquefaction test was successfully done.  Currently preparing for a large prototype test using inverse-compton scattered beam of higher energy γ rays < 40MeV.

25. NuFACT’01 29/05/2001 Tsukuba, Japan R&D works on Liquid Xenon Photon Detector for μ→eγ 25 Summary cont ’ d Refrigerator  Recondensation of 100 liter of liq. Xenon was successful.  Studying other coolants for obtaining better cooling efficiency. Attenuation Length Measurement  Setup construction is going on.  Measurements will start in this autumn at Novosibirsk. Final Detector  Design work just started.  Xenon, γ window, PMT mass-production.