Study of a scintillation counter consisting of a pure CsI crystal and APD

Transcription

1 Study of a scintillation counter consisting of a pure CsI crystal and APD JIN Yifan Aihara Lab August 12th,

2 Outline Belle II calorimeter upgrade Electronics noise in the scheme with APD CsI(pure)+(1-4)APDs light output and equivalent noise energy Improvement of the light output Novel wavelength shifters with nanostructured organosilicon luminophores Summary and Plans 2

3 Belle and Belle II Belle experiment operated at KEKB accumulated the world's largest statistics of B meson decays. CPV was observed in B decays at Belle. CKM mechanism was confirmed. And plenty of other fruitful results were achieved at Belle. At Belle, we are searching for New Physics in many processes (B decays, D decays, τ decays), for example, decay mode B -> τ ν was studied [PRL (2013)]. This mode is sensitive to the BSM with charged Higgs (2HDH) Br B MB 2 rh= = 1 tan 2 Br B SM MH 2 b u W/H+ τ ν Br B = 0.72± 0.27 stat ±0.11 syst The statistical error is still large, so higher luminosity is needed. Belle II, as an upgrade of Belle, planned to increase the luminosity by a factor of 40, will be capable to test several BSM. Belle II is complementary to the current and coming energy frontier experiments (LHC)

4 Belle II detector 4

5 Upgrade of end cap ECL As the luminosity gets increased of 40 times, the pile-up noise produced by the high rate low energy background photons will be very notable and severe, especially in end cap electromagnetic calorimeter (ECL). σe(pile up) ~ Ēγ ν τ Ēγ, average energy of background photon; ν, background event frequency; τ, shaping time Reducing shaping time only will reduce the signal (CsI (Tl)) at the same time. Fast scintillators with as small as possible scintillation decay time are needed. We chose pure CsI, due to its fast scintillation time, moderate cost, radiation hardness and handy mechanical property. Therefore, shaping time is set to be 30ns. A factor of with pure CsI. is earned to reduce the pile up noise However, the light yield of pure CsI is only 10% of that of doped one. Therefore, PIN-photodiode (without internal amplification) is no longer sufficient. New photosensors with internal amplification, like vacuum photopentodes, silicon photo multiplier (SiPM) and avalanche photodiodes (APD), are under consideration. 5

6 End cap ECL upgrade (option 1) Photopentode The main end-cap ECL upgrade option is to use pure CsI crystals readout by Hamamatsu photopentodes R11283MOD-A (ENE ~ 200 kev [1]). However, there are some difficulties: no redundancy, notable dependence on magnetic field, long term stability, new mechanical support is needed. [1] V.Aulchenko, et al. "Electromagnetic calorimeter for Belle II." Journal of Physics: Conference Series. Vol No. 1. IOP Publishing,

7 End cap ECL upgrade Hamamatsu APD S APD Hamamatsu S Ubias = 394 V: Gain = 50, Idark = 8 T=25 oc (1/G)(dG/dT)[%/oC] In the CsI(pure) + Si APD option, APDs from Hamamatsu Photonics are investigated. The advantages of APD are: 1, compactness; 2, insensitivity to magnetic field; 3, demanding of bias voltage and low dark current of ~ na. The main problem with APD is to reach desirable level of electronic noise. With the actual size crystal and 1 APD (1 x 1 cm2) Hamamatsu S , we obtained ENE 2 MeV, while the required ENE 0.5 MeV. 7

8 Photopentode VS APD Photopentode APD S APD S Uoperating(V) > Gain Sensitive area (cm2) QE(%) Capacitance (pf) ~ << 1% 3% 3% The low capacitance and large sensitive area provide photopentode low noise. However, at bias voltage and QE, APD's performance is better. In the case of photopentode, the stochastic noise is the dominant term. However, for APD, the situation is reverse. Electronics noise is dominant, and stochastic noise can be neglected. 8

9 Layout of setup Aluminized mylar CsI crystal 6 x 6 x 30 cm3 Teflon APD CAEN preamp. Shaper CP 4467A Fast Shaping Amplifier (NIM) τ = (20 500) ns ADC PC Hoshin C008 16ch peak hold ADC (CAMAC) 4ch preamplifier CAEN A1422B045F3 45 mv/mev (1 V/pC) 9

10 Study of electronic noise ENC2 = A2 τ + (B2/τ + E2)C2 + D2 Shot noise Thermal noise 1/f noise Shot noise coefficient, A = 2 Idark K1 g F/e Thermal noise coefficient, B = 4 kb T RS K2 e 1/f noise coefficient, E= K3 Af Af is a noise coefficient of order V2 Additional noise e positron charge; Id dark current; F excess noise factor; C APD junction capacitance; τ shaping time; g APD gain; K1 K2 K3 shaper factor; τbest = BC/A ENC2best = 2ABC + E2C2 + D2 RS equivalent serial resistance of preamp D additional noise. 10

11 Measurement of total ENC and addition noise (D) Cfb U bias Rfb Rb τ= ns Rs Shaper Ccal 1850 ADC PC test channel 10ms us u0 After preamp After shaper = 6% = 21% At the shaping times from 20 ns to 500 ns, D is not constant. It varies strongly, which is explained by the relatively large additional parallel (ina) and serial (ena) noises. Fast shaper of better quality (like ORTEC 474, 579) might be helpful to decrease D 11

12 Measurement of thermal noise B, 1/f noise E Two well known capacitors C1 and C2 were used to measure B and E. B2/τ + E2 = (Q12 Q22)/(C12 - C22) Cfb B = (26.2 ± 0.8 ± 4.8) ns/pf U bias Rfb Rb E = (6.1 ± 0.1 ± 0.4) 1/pF τ= ns Rs Shaper Ccal test channel ADC PC u0 B2/τ = (4kBK2TRS)/τe2 2 BF862 FETs RS, equivalent serial resistance, it is dominated by reversal transconductance of the CAEN preamp's FET (BF862) RS 50 Ω was also measured with additional serial resistance at the CAEN preamp input. Therefore, we tried FET 2SK932-23, one of the best FET at short shaping times, with intention to get lower RS. However, no obvious improvement is obtained. There is no potential to reduce B and E! 12

13 Shot noise, excess noise factor F Q2no Iphoto=2 e Id τ g F K+(B2/τ+E) Cd2+D2 Q2with Iphoto=2 e (Id+Iphoto) τ g F K+(B2/τ+E) Cd2+D2 F = (Q 2 with Iphoto -Q 2 no Iphoto )/(2 e Iphoto τ g K) K2(EXP) = 0.44 ± 0.02 K2(CR-4RC) = 0.45 measured by PIN-photodiode Cfb Ubias Rfb Rb Rs Shaper Iphoto APD ADC Ccal test channel PC u0 S : g = 50, F = 5.1 ± 0.5 S : g = 50, F = 3.4 ± 0.4 Excess noise refers to the additional noise due to avalanche fluctuation, and can be expressed as: F =h*g+(2-1/g)(1-h) where h is the ratio of the hole impact ionization rate to that of electrons.13

14 ENC vs. shaping time ENC2 - D2 = A2 τ + (B2/τ + E2)C2 A = (2 Idark K1 g F/e) B = (4 kb T RS K2) /e E= (K3 Af C2) Discrepancy between calculated and measured ENC2-D2 is due to the uncertainty in CAPD The agreement between the measured noise and noise calculated by the formula indicates good suppression of the correlated noises 14

15 Light output (LO) and ENE Cosmic muons are used to calibrate ADC channels in units of energy (MeV) CsI 6x6x30 cm3 MC Acosmic EXP ConversionADC(MeV/ch) = EMC /A peak peak Epeak(cosmic) 33 MeV ENE = σcal ConversionADC Cfb Ubias The light output is measured by comparison of the signal from cosmic muons (A ) with calibration Rfb Rb cosmic signal (A ). Acal Rs cal Iphoto Ncosm(ph.e.) = (Ccal U0 / e) (Acosm / Acal) 2 Sensitivity = Ncosm/Epeak/(APD gain = 50)/(SAPD [cm ]) MC Sensitivity = ph.e. / MeV / cm2 Shaper APD Ccal ADC PC test channel u0 15

16 ENE, several APDs per crystal S S Total noise=apd noise + D 1 APD S has essentially larger dark current (26 na) in comparison with the average one (8 na), we introduce correction to ENE. ENE Measured Estimated 2 S (same Idark) MeV MeV 2 S MeV MeV 4 S MeV MeV The ENEs of the counter are far away from our goal. Further studies are needed. 16

17 To increase the light output The light collection coefficient is strongly depending on the quality of APD coupling to crystal and reflectivity of the wrapping material. 1, Three types of optical grease were tested, OKEN-6262A, BC-630 and TSF451-50M. However, we didn't find anything better than OKEN-6262A. 2, White porous Gore-Tex teflon was confirmed as the best reflector at UV range [1]. The thickness of the white teflon was studied. It is shown that 200 um is sufficient, further increase on thickness provides no more than 5% improvement on signal. 3, Novel wavelength shifting (WLS) plates containing nanostructured organosilicon luminophores [2] provides essential increase of light output. [1] M.Janecek, IEEE Trans. Nucl. Sci (2012) 490. [2] S.A. Ponomarenko, et al., Scientific Reports 4 (2014)

18 Wavelength shifters with organosilicon luminophores Based on the nanostructured organosilicon luminophores (NOL-9,10,14) from LumInnoTech Co., the WLS plates were developed ((60 x 60 x 2) mm3). 302 nm 502 nm PLQY=90% 327 nm 588 nm PLQY=95% 337 nm 655 nm PLQY=78% The absorption and emission spectra of these NOL's match our needs very well (λcsi = 320 nm). The improvement of the APD QE is by a factor of

19 Results with WLS plates With optical grease It is important to cover the whole 6 x 6 cm2 plate by optical grease! CsI(pure) WLS APD NOL-9 turns out to be the best WLS that provides an enhancement on signal by a factor of about 3. 19

20 Results with WLS plates Without optical grease However, the largest signal is achieved w/o optical grease between crystal and WLS plate! CsI(pure) WLS APD ENE(2 S ) = (0.53 ± 0.05) MeV ENE(4 S ) = (0.45 ± 0.05) MeV (one APD was operated with gain>50) Error originates from the relative temperature gain variation ((1/G)*(dG/dT)), accuracy of simulation of cosmic peak position and statistical accuracy of the data. 20

21 Summary & Plans Hamamatsu APDs of S8664 series provide a promising option for Belle II end cap ECL upgrade. Several APDs per crystal allow us to decrease further ENE and provide readout redundancy. Essential increase of the light output of the CsI(pure)+APD(s) counter was achieved with WLS plates based on the nanostructured organosilicon luminophores (NOL-9). The ENE of the counter with 2 S APDs was measured to be ENE = (0.53 ± 0.05) MeV, which is close to project requirements. Radiation hardness of WLS plates is under investigation. We are testing the scheme where APDs are mounted to the side edge of the thick WLS plate. We will test pipeline readout scheme with a special shaper-fadc board. The preliminary results have been already reported at 13th Pisa Meeting and PhotoDet-2015 conferences. 21

22 Acknowledgements Finally, I would like to thank Prof. Aihara and Dr. Epifanov for their instruction on this study. 22

23 Back up 23

24 Evaluation of the desirable noise level (0.5 MeV) Energy resolution can be expressed as: shower leakage electronics noise small additional noise from: stability of counter, stochastic noise and non-uniformity. Taken into account the typical threshold of the energy of photon in data analysis of Belle, 100 MeV, the first term gives an energy resolution 3.4%, in other words, σe(shower) = 3.4 MeV. Electronics noise contributes < 10% total noise (1.13 MeV) At Belle, a 100 MeV photon shower involves five crystals. And we assume these channels are uncorrelated. 24

25 Improvement of the LO Refraction index Light collection efficiency OKEN-6262A nm from the producer nm) 85% TSF451-50M nm) 98% % BC-630 Three types of optical grease were tested ( = 100 μm), OKEN-6262A provide the largest light output Effect of the thickness of white porous Gore-Tex teflon was studied, thickness of 200 μm was found to be optimal. signal amplitude 600 M.Janecek, IEEE Trans. Nucl. Sci (2012) um 185 um 2*122 um 500 um thickness of teflon 25

26 Characteristics of APD APD is a dominant source of the signal temperature variations, which have to be compensated (1/G)(dG/dT)[%/oC] To compensate temperature variations of APD gain, we can organize temperature sensor - bias voltage feedback. 26