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Line 28: − [[Image:PulserCircuit.png|frame|A pulser circuit designed to drive the LED for photo-detector bench tests. The circuit differentiates the input signal, thus the amplitude of the input square wave controls pulse amplitude up to the saturation point, at which the pulse shape broadens to a maximum of 6 ns. V<sub>s</sub> = 5 V]]+ + + + − A wooden dark box was initially constructed for the a hybrid photo-diode (HPD) module DEP PP0350. A pulser was installed on the opposite end of the box composed of a fast LED driven by a pulser circuit shown on the right. + − + Line 51:
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→Bench Test Setup
=== Hardware ===
=== Hardware ===
A fast light source operating in an environment with little background is necessary for the tests described here. The challenge is in preventing light leaks in this chamber despite the need for access ports, patching wires through walls and installing sensor and temperature control modules that must interface with the outside.
It is also preferred that the light source is fast and close in wavelength to the light from scintillating fibers, which are in the blue-green.
==== Dark Box Construction ====
Running this setup with an HPD of [[:Image:S20photocathode_QE.jpg | known characteristics]] allows us to calibrate the light intensity. The SiPM detection efficiency can then be characterized relative to the HPD. The signal from the SiPM was clean enough to distinguish peaks corresponding to discreet photon (pixel) counts in the histogram signal integrals. Therefore, the gain of the SiPM was found independently of the HPD - the SiPMs were self-calibrating! The [[#SiPM_Measurements|measurements section]] below describes this feature further.
Running this setup with a hybrid photodiode (HPD) module DEP PP0350 of [[:Image:S20photocathode_QE.jpg | known characteristics]] allows us to calibrate the light intensity. The SiPM detection efficiency can then be characterized relative to the HPD. The signal from the SiPM was clean enough to distinguish peaks corresponding to discreet photon (pixel) counts in the histogram signal integrals. Therefore, the gain of the SiPM was found independently of the HPD - the SiPMs were self-calibrating! The [[#SiPM_Measurements|measurements section]] below describes this feature further.
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==== Light Pulser ====
[[Image:PulserCircuit.png|frame|A pulser circuit designed to drive the LED for photo-detector bench tests. The circuit differentiates the input signal, thus the amplitude of the input square wave controls pulse amplitude up to the saturation point, at which the pulse shape broadens to a maximum of 6 ns. V<sub>s</sub> = 5 V]]
A pulser circuit was designed with a pulse height controlled by the amplitude of a step function from a function generator. The pulser differentiates the step function signal and therefore can create pulses as narrow as the rising edge of the step function. Above a saturation point, the pulse broadens to a maximum of 6 ns. The adjacent figure shows the pulser circuit that drives the LED. The LED has some finite rise time and sometimes a very long decay tail. This response function convolves the pulser signal so the speed of the combined system has to be analyzed for each LED type and evaluated for the use of photon detector characterization.
===== Choice of Light Source =====
The original LED available to the group was the blue QT Optoelectronics MV5B60. The calibrating this device with the HPD showed a very long decay of its pulse. Several more devices with larger wavelengths were procured and measured settling on a yellow LED (Fairchild MV8304) for its speed. The SiPMs were characterized with respect to one another and the HPD using this device. With a mean wavelength of about 590nm, this LED is near the peak of the SiPM detection spectra.
The original LED available to the group was the blue QT Optoelectronics MV5B60. The calibrating this device with the HPD showed a very long decay of its pulse. Several more devices with larger wavelengths were procured and measured settling on a yellow LED (Fairchild MV8304) for its speed. The SiPMs were characterized with respect to one another and the HPD using this device. With a mean wavelength of about 590nm, this LED is near the peak of the SiPM detection spectra.