Advancement in development of photomultipliers dedicated to new scintillators studies.

Transcription

1 Advancement in development of photomultipliers dedicated to new scintillators studies. Maciej Kapusta, Pascal Lavoutea, Florence Lherbet, Cyril Moussant, Paul Hink

2 INTRODUCTION AND OUTLINE In the validation and verification processes of new scintillators development, PMTs are used to study and measure inter alia: (1) scintillators luminescence and scintillators emission spectrum, (2) scintillation pulse rise and decay time, (3) scintillator light output, (4) scintillator linearity, (5) scintillator energy resolution.

3 Scintillators luminescence and scintillators emission spectrum Thermo- or Radio- luminescence is a superb tool widely used in scintillator crystals studies. The excellent detection sensitivity of such a systems exists when the emission of the studied sample occurs in short wavelength region (e.g. around 400nm) because PMTs are most responsive in this region. Ideal PMT for such detection system should be characterized by: 1.) Minimized differences in PMT response for blue and red light 2.) Very high Signal-to-Noise ratio 3.) Very good linearity and stability of gain New development in the bi-alkali photocathodes improved their quantum efficiency but also increased dark current noise. By generating of multiple photon interactions of light with the guide by reflections within a waveguide structure it is possible to raise the absorptance of PC, and hence the QE.

4 RED new PMT for spectrometers Prism launch Edge lunch No reflection loss at input For a luminescence studies new PMT based sensor was prepared. It is characterized by broad spectral response covering the nm wavelength range due to its unique fiber-optic input. This sensor has improved signal-to-noise ratio.

5 50 45 edge lunch Quantum efficiency [%] prism launch Wavelength [nm] By increasing the probability of interaction with PC the QE of PC was increased while dark current remained the same.

6 Signal to Noise: Cathode response at 800 nm High first dynode gain Electron Optic First Dynode Gain Impact on Cathode Response 0.14 QE Curve Responsivity (A/W) X 1st Dynode Gain 5X 1st Dynode Gain 15X 1st Dynode Gain Elements of high first dynode gain: High accelerating voltage PC to Dy1 High emission ratio in situ evaporation of Sb Wavelength (nm)

7 Stability Test ua 1 ua 2.5 ua 10 ua GAIN STABILITY Improved insulators New dynode shape Process changes Serial number 907 uses improved process for reduced gain shift at higher current levels. Responsivity (ma/w) Temperature (degrees C) 500 nm 550 nm 600 nm 650 nm 700 nm 750 nm Temperature Stability of the Photocathode Negative 0.4% per degree C for short WL Positive 0.4 % per degree C for long WL Neutral at 650 nm

8 Rise and decay time of scintillators light pulse Common method to measure the scintillation light decay is to observe the PMT anode pulse directly on the oscilloscope. In such a case time distribution spectrum combines the effect of the illumination function and the instrumental response function as fallows: F(t j )=i(t)*f p (t) where: f p (t) - instrumental response function i(t) illumination function For single exponential decay this equation become: N total number of «counts» - decay time constant F(t j )=N f p (t)e - t dt where: In the case of scope measurements the instrumental response function can be measured as a width of SER of PMT.

9 Rise and decay time of scintillators light pulse FAST PMTs: - Low width (FWHM) of the SER function -Fast rise time of the pulse for single-photon PHOTONIS XP3060 HV = 1300 V Rise time: 2.1 ns SER width 3.1 ns FWHM HV = 2000 V Rise time: 1.6 ns SER width 2.7 ns FWHM

10 Rise and decay time of scintillators light pulse LaBr 3 1 x 1 XP3060 HV=900V HV=1300V =16.5 ns =15.9 ns HV=1600V HV=2000V =15.9 ns =17.6 ns

11 Rise and decay time of scintillators light pulse PMTs could be used for decay time measurements of the scintillation pulse. Low contribution of PMT assured by narrow width of SER Fast rise time of SER Linearity Family of XP20D0, XP3060, XP1020. The system response time is still to slow to use PMT for scintillator light pulse rise time measurements. PHOTONIS development group is working on this problem to provide community with new super fast sensors suitable for rise time of scintillation pulse measurements.

12 Energy resolution and non-linearity The development of new LaCl 3 and LaBr 3 crystals is challenging again the PMT performance level. New requirements are addressed to the linearity of the PMT anode signal and the PMT contribution to the energy resolution. Ideal case σ = E N N γ γ = 1 N γ Nγ QE Number of incident photons Quantum Efficiency Reality CE Collection Efficiency: σ ENF ENC = + E Nγ QE CE Nγ QE CE G PMT Poisson statistics, no intrinsic no scintillator Electronics 2 ENF ENC G Excess Noise Factor (from Dynodes) Equivalent Noise Charge (Readout Noise) Gain

13 Energy resolution and non-linearity Key PMT characteristics for good PHR: (1) high QE of PC, (2) high and uniform secondary gain 3 PMTs type XP5200 with the QE of 37%, 34% and 30% for 420 nm Avg. gain per stage = 5.8 Avg. gain per stage = 3.83 Avg. gain per stage = 4.20

14 Energy resolution and non-linearity TULIP SHAPE PMT, XP stages, 55mm tube = 5.8 QE = 30% Reflection due to the shape of the PMT envelope

15 Energy resolution and non-linearity XP20C2, 8 stages, 39 mm tube XP2A508, 8 stages, 28 mm tube

16 Energy resolution and non-linearity How to calculate contribution of PMT to the energy resolution? The statistics of PHR distribution (FWHM) involve both the crystal and the PMT processes and can be expressed as: FWHM 1 1+ δ 1 = Δ N phe 2 PMT scintillator In 1966 G.H. Narayan and J.R. Prescott showed that: PMT statististics dominate energy resolution at low gamma-ray energies Scintillator statistics dominate it at high gamma-ray energies IEEE TNS Vol. NS-13, No.3, pp , 1966.

17 Conclusions Quantum efficiency of PMT photocathode is of PRIME importance for accuracy of measurements. The high QE can be archived by increasing probability of interactions of photons with PC. RED new sensor is utilizing this feature. QE of 48% was obtained for blue light without increase the PMT noise. The new class of fast PMTs could be used for scintillation pulse decay measurements by means of scope due to their narrow SER function and fast rise time. Although QE is of prime importance to obtain perfect PHR the high secondary emission yield especially in the first stage of PMT is necessary. Due to improved PMTs construction, dynodes shape and configuration of the last stage the excellent linearity and gain stability was achieved. Tulip shape of the PMT glass envelope increase effective QE The influence of PMT on the PHR could be calculated from basic parameters.

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