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Characterization of a Microchannel Plate Photomultiplier Tube with High Sensitivity Gaas Photocathode

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Characterization of a Microchannel Plate Photomultiplier Tube with High Sensitivity Gaas Photocathode 1 Characterization of a Microchannel Plate Photomultiplier Tube with High Sensitivity GaAs Photocathode John Martin, Paul Hink Abstract - The characteristics of an 18mm photomultiplier tube (PMT) having a high sensitivity GaAs photocathode have been studied. This PMT, the BURLE 85104, uses a dual microchannel plate (MCP) electron multiplier. We report measurements that include standard DC response, single electron sensitivity and resolution, time response, pulse and count rate linearity, and dark counts as a function of temperature. Data is also presented for a gated version of the PMT that has a high-speed anode. The and Peak:Valley Ratios of greater than 2:1, properties of these MCP-PMTs make them the 85104 is an effective low noise photon well suited for applications such as LIDAR, counting tool. fluorescence microscopy and chemiluminescence. Figure 1 illustrates the basic operation of an MCP PMT. Photons pass through the I. Introduction: faceplate of the PMT and initiate the release A new family of Microchannel Plate of photoelectron(s) from the photocathode Photomultipliers (MCP PMT) broadens our surface on the interior side of the faceplate. existing choice of PMTs. The 85104 tube The photoelectron is accelerated toward the st type offers exceptional spectral response, front surface of the 1 MCP due to the 300V uniformity and speed that is attractive for a bias between the cathode and MCPin.. The number of applications, including . LIDAR, photoelectron collides with the interior edge fluorescence microscopy and of a single pore, creating more electrons, chemiluminescence The compact design which are consequently accelerated through with integrated VDN and optional gating the pore structure of the MPC due to the capability makes the 85104 a powerful plug- 3000V bias on the double stack of MCPs. and-play solution. The 85104 features a Continued collisions with channel walls and GaAs photocathode with greater than 1250 electron acceleration yield a charge cloud uA/lm luminous sensitivity with excellent that is emitted from the back side of the red sensitivity having greater than 20% MCP stack and accelerated toward the anode quantum efficiency at 850nm. With Single collection area which remains at ground Photoelectron Resolution better than 150%, potential. 2 photon Typical Gain Curve Faceplate Cathode 1.0E+07 Photoelectron ∆V ~ 300V MCPs ∆V ~ 3000V 6 1.0E+06 Gain ~ 10 ∆V ~ 300V Tube Gain Anode 1.0E+05 2000 2200 2400 2600 2800 3000 3200 -Figure 1- MCP Voltage (V) -Figure 3- II. Cathode Sensitivity: The GaAs transmission mode cathode has a IV. Single Photoelectron Resolution: wide spectral range with high Quantum The combination of the simple design and Efficiency from 400-900nm (see figure 1). choice of high quality MCPs, the 85104 is a low noise device that offers good single Typical Spectral Response photoelectron resolution. Figure 4 illustrates 1000 100 a typical pulse height spectrum for the 85104. The plot shows data for the tube run under dark conditions as well as a 100 10 subsequent run with low level single photon light levels. The typical Peak:Valley Ratio is in excess of 2:1, with a Pulse Height 10 1 Resolution (Peak/FWHM) for the Single PE Peak of less than 150%. This particular tube has a SPE Resolution of 134%. Under o 1 0.1Quantum Efficiency (%) cooled conditions (-30 C), the 85104 has Cathode Sensitivity (mA/W) 300 400 500 600 700 800 900 1000 less than 200 cps dark noise. Wavelength (nm) Typical Single PE Spectrum -Figure 2- 40 III. PMT Gain: 35 Signal & Dark Counts The electron multiplier in the 85104 is a 30 double stack of 5um pore MCPs with L/D 25 Ratio of 60:1. The MCPs are aligned in a 20 chevron configuration to eliminate the 15 Dark Counts possibility of ion feedback to the sensitive Count Rate (cps) 10 photocathode surface. The duel MCP 5 design provides high, uniform electron 0 0.00E+00 5.00E+06 1.00E+07 1.50E+07 multiplication well in excess of 1M gain Electron Gain (see figure 3). -Figure 4- When the PMT is run at higher voltages and exposed to slightly higher light levels (statistically implying the presence of 1, 2, 3PE’s), the 85104 is able to qualitatively 3 distinguish 1PE, 2PE and 3PE peaks (see figure 5). (Note: The dark count rate is determined by recording the total number of dark counts between Single Electron Spectrum (3300V) 1/3 photoelectrons and 3 photoelectrons divided by 2000 the time of acquisition (typically 100s).) 1500 VI. Timing Characteristics: The speed of the Microchannel Plate multiplication, short electron path lengths 1000 and high speed anode design provide for a Counts very fast, clean signal from the 85104. 500 Transit times of less than 1ns. Single photoelectron pulses have rise times of less 0 than 200ps, and fall times of less than 1ns. 0 5e+6 1e+7 2e+7 2e+7 The 50-Ohm impedance output of the high Charge (e) speed anode makes the 85104 compatible -Figure 5- with a variety of high speed processing V. Dark Noise Measurements: electronics. Figure 7 illustrates a single PE The semiconductor III V nature of the GaAs pulse. An ongoing effort is being made at cathode demands cooling to reduce Burle to further improve the anode design to thermionic emission of free electrons from reduce the FWHM and make the fall time of the surface of the cathode. At room the pulse comparable to the rise time. The temperature (22oC) the typical dark count goal is to have Rise Time = Fall Time = rate of the 85104 is 10,000-20,000 CPS. 200ps. When the PMT is pulsed with a 28ps Under cooled conditions, the dark noise wide pulsed laser pulse with effective light dramatically drops to less than 200 CPS at - levels on the order of several hundred 30oC. Figure 6 illustrates the decrease dark photoelectrons, the rise time of the resulting count rate as a function of temperature. output pulse increases to ~350ps. This Though the standard dark count rates test is indicates that the Transit Time Spread (TTS) performed at -30oC, figure 5 reveals that for multi-photon events is on the order of there is little advantage gained by cooling 200ps. This is essentially due to the below -10oC. structure of the GaAs cathode. To date, a Dark Count Rate vs. Temp comprehensive test of TTS has not yet been performed. 10000 1000 100 Dark Count Rate (cps) 10 -40 -20 0 20 40 Temp ( OC) -Figure 6- 4 Typical DC Linearity 110.0% 100.0% 90.0% 80.0% Linearity (%) 70.0% 0 100 200 300 400 500 Anode Current (nA) -Figure 8- VIII. Count Rate Linearity: -Figure 7- Any single channel pore in an MCP PMT Rise Time = 150.6ps Fall Time = 951.9ps can remain ‘dead’ after a pulse for as long as FWHM = 1.128ns. several milli-seconds as charge is slowly restored. However, the 18mm diameter of VII. Pulsed and DC Linearity: the MCPs effectively provides millions of MCP based PMTs are not as robust as their discrete multipliers in the PMT. Therefore metal channel dynode counterparts so if illumination is spread across a wide area special attention needs to be given to protect of the cathode, it is unlikely that any one the PMT from over exposure to light. The pore will be required to multiply charge maximum DC output level recommended during this recharge period. Hence, the for the 85104 is 300nA, though life tests 85104 has very good count rate linearity. have been performed for sustained periods Figure 9 illustrates the multiplication in excess of 0.9uA. Pulsing the 85104 with process through a single MCP pore. a high speed laser at a low duty cycle (Ioutput <300nA) provides 95% pulsed linearity at Photoellectr 400mA peak pulse output. Figure 8 is a representative DC linearity curve for the 85104. Linearity for MCP PMTs are governed by a combination of the strip current of the MCPs and the pre size of the channels in the MCPs. Smaller pore sizes MCP’ provided a greater number of effective multiplier paths pre unit area, while higher strip currents enable spent channels to be recharged more quickly. Advances in MCP Exiitiing technology at Burle in both of the above will Electron result in MCP PMTs with higher pulse, DC Electron and count rate linearity. - Figure 9- Figure 10 illustrates the typical count rate linearity for single PE events when the PMT 5 is operated at 0.5x106 gain. In summary, the Anode Output vs. Extracted Charge 85104 remains 93% linear at count rates of 2x106 CPS. (Note: the count rate is determined by the total 100% number of counts above 1/3 photoelectrons over the time of acquisition.) 90% 80% Count Rate Linearity 70% 110.0% Anode Output 60% 50% 100.0% 0.0 0.5 1.0 1.5 2.0 2.5 90.0% Extracted Charge (C) Linearity (%) 80.0% -Figure 10- 70.0% 0.E+00 5.E+05 1.E+06 2.E+06 2.E+06 Count Rate (CPS) Figure 10 shows a typical life test run on the 85104 where the PMT was run at 1x106 gain -Figure 10- under constant light exposure yielding an initial DC output of 300nA. The output was IX. Lifetime Data: tracked for nearly 2200hrs of continuous A classic concern of MCP PMTs has operation. The overall anode output focused on the area of lifetime, namely Ion dropped 28% with a total extracted charge Contamination of the photocathode. In of 2.2C. Following the data trend, one standard metal channel dynode PMTs there would expect about 5-7C of extracted charge is a chance of residual gas ionization in the before the tube realizes a 50% decrease in back end of the PMT where the electron anode output signal.
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