A METHOD TO ENHANCE THE SENSITIVITY OF PHOTOMULTIPLIERS FOR AIR CHERENKOV TELESCOPES

Transcription

1 A METHOD TO ENHANCE THE SENSITIVITY OF PHOTOMULTIPLIERS FOR AIR CHERENKOV TELESCOPES D.Paneque, H.G. Gebauer, M. Martinez, K.Mase R. Mirzoyan, A. Ostankov, T.Schweizer David Paneque M.P.I. Munich (Germany)

2 OUTLINE 1- Introduction 2- QE increase by applying a lacquer - UV sensitivity enhancement by use of a WLS. - QE enhancement by a scattering layer 3- Increase in the DE of an IACT 4- Additional enhancement in the QE by the use of light collectors with hemispherical window PMTs 5- Conclusions 2

3 1- INTRODUCTION. THE MAGIC TELESCOPE!MAGIC is a Cherenkov telescope that will be installed at the Canary island La Palma (Spain) in October 2002! Alt azimuth mount. Range: 400º in φ, 180º in θ.! Carbon fiber space frame construction! 17 m parabolic mirror with f/d =1! MAGIC aims to cover the unexplored energy window from 30(12) GeV to 200 GeV in gamma ray astrophysics Phase 1; EMI 9116A, 9117A from ETL - Rb-bialkali Photocathode - Borosilicate hemispherical window! High conversion of Cherenkov photons to photoelectrons is mandatory in reducing the Energy threshold (Eth) E th A mirror 1 LCE LCE = R LC QE CE

4 2- QE ENHANCEMENT BY APPLYING A LACQUER Cherenkov spectrum extends down to 290 nm UV sensitivity is required in our PMTs 2.1- UV sensitivity increase by a WLS! Vacuum deposition technique - Necessity of vacuum-coating unit - Coating with weak mechanical resistance! Eigen & Lorenz (NIM A 167 (1979) 405): PMT dipped into solution of WLS +plastic binder+organic solvent - Easy and cheap implementation - Excellent mechanical resistance - Good optical contact with high internal trapping of the re-emitted light! Requirements for being used in MAGICT - WLS absorption below 310 nm and re-emission close to 380 nm - Decay time about 1ns 1,4 p-terphenyl (PTP) + Paraloid B-72 Abs. below 315 & em. at 340 nm Decay time; 0.9 ns Dichloromethane or Toluene used as solvent Very satisfactory results obtained with 0,5 gr. PTP + 2 gr. B ml. Dichloromethane

5 ! Setup for the measurement of the QE(λ) - Spectrofluorometer; N(λ) - PMT operated as photocell; I k (λ) - Calibrated PIN diode; I d (λ), QE d (λ)! Results I k ( λ ) QE PMT ( λ ) = QE d ( λ ) I ( λ ) d 5

6 ! Setup for the measurement of the QE(λ) - Spectrofluorometer; N(λ) - PMT operated as photocell; I k (λ) - Calibrated diode; I d (λ), QE d (λ)! Results obtained I k ( λ ) QE PMT ( λ ) = QE d ( λ ) I ( λ ) d 6

7 ! Setup for the measurement of the QE(λ) - Spectrofluorometer; N(λ) - PMT operated as photocell; I k (λ) - Calibrated diode; I d (λ), QE d (λ)! Increase in the U.V. I k ( λ ) QE PMT ( λ ) = QE d ( λ ) I ( λ ) d 7

8 ! Setup for the measurement of the QE(λ) - Spectrofluorometer; N(λ) - PMT operated as photocell; I k (λ) - Calibrated diode; I d (λ), QE d (λ) I k ( λ ) QE PMT ( λ ) = QE d ( λ ) I ( λ )! Further increase in the sensitivity!! d 8

9 2.2- QE increase by a scattering layer! A possible way of increasing the QE of a PMT QE ph. P absorbed P e escape Optimum thickness for a given wavelength range QE can be raised if path length is increased for the photon but not for the electron -For photons with large angle of incidence 15-20% QE increase for 60 0 at 600 nm -Window treatment to scatter light in a commercial PMT (frosted window) PMT type 9829 (Electron Tubes) 7% QE increase at 400 nm 17% QE increase at 600 nm Problems to keep the glass clean 9

10 ! Simple way of producing a scattering layer - Increasing the quantity of Paraloid B-72 by a factor Dipping the PMT 2-3 times (waiting 5 min.) 10

11 - Good homogeneity in the milky layer Max differences in photocurrent < 5% (<3% in the non-coated PMT ) - Detailed study of the uniformity in the response vs illuminated position on the photocathode is ongoing 11

12 ! Sensitivity increased in all wavelengths 12

13 ! Sensitivity increased in all wavelengths Milky layer increases QE more for longer λ WLS (PTP) increases QE below 330 nm λ= 300 nm Increase > 80% λ = 350 nm Increase > 10% λ = 400 nm Increase > 15% λ = 500 nm Increase > 20% λ = 600 nm Increase > 50% 13

14 ! A simple model - Larger quantity of reflected light - Some photons may be slightly deflected - Some photons may be scattered enough to cross photocathode (PhC) twice - Others may get trapped in PhC-coating - Some photons reflected off in the PhC may be scattered back Net effect is a substantial QE increase in the entire spectrum!!! 14

15 ! Optimization of the quantities Variations in quantity of P.B-72 for 0,5gr of PTP in 50 ml of Dichlor. More than 3 gr. needed to form frosted layer No big differences found, but for a small decrease in the U.V. for 10 gr. Variations in quantity of PTP for 5gr of P.B-72 in 50 ml of Dichlor. PTP important only below 330 nm The more PTP, the easier the frosted layer is obtained (PTP acts as local seed) Fast evaporation of solvent is needed 15

16 ! QE increase for 4 EMI 9116A type PMTs coated with 0,5gr PTP+5gr P.B-72+50ml Dichloromethane Differences in sensitivity Increase smaller than spread in QE itself! QE increase in a flat UV transmitting window PMT QE increase substantially smaller for flat window 8% at 400 nm 19% at 600 nm 16

17 3- INCREASE IN DETECTION EFFICIENCY (DE) OF AN IACT AT 2200 m a.s.l.! Cherenkov photon spectrum expected from 10GeV-30TeV showers at 2200 m a.s.l! Increase in the effective QE (QE eff ) QE eff N N phe ph ( ZA ) ( ZA ) = QE ( λ ) N ( ZA, λ ) dλ N ( ZA, λ ) dλ Coated PMTs increase QE eff by 21%!! -Small ZA dependence - Numbers calculated for PMT 1930 E Same effect in the telescope E th as the increase in the mirror area by 21%!! (i.e. from 234 m 2 to 283 m 2 ) th A mirror R mirror 1 Eff LC QE CE 17

18 4- INCREASE IN THE QE BY ALLOWING LIGHT TO CROSS THE PhC TWICE IN A HEMISPHERICAL PMT! Light Collectors (LC) are needed in IACT telescopes! LC+PMT with hemispherical PhC allows photon trajectories to pass the photocathode twice! Estimation of this effect has been measured Double cross non-coated PMT at 400 nm QE Double _ cross Non Coated QE 1.20

19 4- INCREASE IN THE QE BY ALLOWING LIGHT TO CROSS THE PhC TWICE IN A HEMISPHERICAL PMT! Light Collectors (LC) are needed in IACT telescopes! LC+PMT with hemispherical PhC allows photon trajectories to pass the photocathode twice! Estimation of this effect has been measured Double cross coated PMT at 400 nm QE Double _ cross Coated QE

20 5- CONCLUSIONS! A simple method consisting in the application of a specially treated lacquer containing WLS has been found to increase substantially the QE of PMTs which are not optically coupled to their light sources.! In the particular case of the EMI 9116A PMTs (hemispherical window and Rubidium-bialkali photocathode) used in an IACT, this technique produces an increase in the Detection efficiency (DE) > 20%! Further enhancement in DE is obtained in combination with light collectors that make some photons trajectories cross the PhC twice! Studies of the uniformity in the response as well as the stability of this coating are ongoing 20