Physics307L:People/Allen/Lightspeed

Objective

This lab calculates the speed of light by measuring the differences in the time interval between when a signal is transmitted and when it is received a short distance away, as the distance between the emitter and the receiver is changed by regular increments. As the distance is decreased, the intensity of the signal at the receiver, a photomultiplier (PMT), is greater, resulting in a higher amplitude output from the PMT to the time-to-amplitude converter (TAC). The TAC is triggered when a pulse strength passes a certain threshold, which is reached more quickly by a higher amplitude pulse. This results in a decrease in the recorded time intervals with respect to the actual time intervals as the emitter, a light emitting diode (LED), and the PMT are moved closer together ~ a phenomenon known as time walk. To compensate for this, there are two polarized lenses placed between the LED and the PMT which can be rotated with respect to each other, to adjust the light intensity before each time interval measurement and keep the amplitude of the output signal from the PMT constant.

Equipment

The equipment is as follows:

• Tektronix Oscilloscope (Model TDS 1002)
• Bertan Power Supply (Model 215, 3000V, 5mADC)
• Canberra Delay Module (Model 2058)
• Ortec TAC/SCA Module (Model 567)
• Harshaw NIM Bin (Model NQ-75)
• Harrison Laboratories Power Supply (Model 6207A, 160V, 0.2A)
• Photomultiplier Tube (PMT)
• LED circuit
• BNC Cables

These are from David Weiss' notes from when we worked together two years ago. There is now a longer cable from the PMT to the TAC, so I did not need to use the delay module in collecting my data this time.

< User:David J Weiss | Notebook

Safety

~ The PMT is connected to a high voltage power source set to near 2400V. Be sure all cables are insulated and not worn.

~ Do not handle the 200V power source to the LED when the power is on, as has exposed contacts in the back and two exposed wires in the front with worn insulation.

~ Do not expose the PMT to ambient light when the high voltage power source is on, or it will be destroyed by the high current passing through it due to the photomultiplier.

Set-Up

The PMT is set inside one end of a 3-4 meter long cardboard cylinder, with a polarizing lens attached to the receiving end, such that this assembly can be rotated with respect to the cylinder. An LED, mounted on a long rod consisting of a series of meter sticks, is located at the other end of the cylinder so it can be moved by measured intervals of distance with respect to the PMT. The LED also has a polarizing lens attached to it between it and the PMT. By rotating the PMT to adjust the angle of alignment of the two polarizing lenses, the intensity of the light pulse received by the PMT from the LED can be attenuated, and kept nearly constant as the distance is changed.

The LED is connected to the 200V power supply and then to the start input of the TAC. The PMT is connected to the high voltage power supply and then connected to the stop input of the TAC. Both connections to the TAC are with BNC cables of similar lengths, that carry pulses when the light flash is transmitted and when it is received. The Tac output is a signal with a voltage level proportional to the start-stop time interval. The output from the TAC is connected to the oscilloscope to be measured and displayed. The output from the PMT is also directly connected to the oscilloscope to measure the intensity of the light received by it. This intensity can then be kept nearly constant by rotating the PMT to change the alignment of the lenses.

Data

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