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Goal

Determine appropriate amplifier for a photomultiplier tube to be used in a detector, so as to prevent the background noise from drowning out the signal from the photomultiplier tube.

Challenges

Slew Rate

The slew rate is defined as the measurement of how fast an amplifier can respond to an electrical signal. Thus, the slew rate determines the range of frequencies that can be amplified according to the formula S = 2πfV where f is frequency and V is peak voltage. In order for our amplifier to work with the photomultiplier tube, the slew rate must be high enough to accurately amplify the signal. The slew rate is expressed in volts per second, but more commonly volts per microsecond. The two amplifiers we considered are the LM 741C Operational Amplifier and the LMH 730154 Differential Amplifier.

Cost

While not necessarily a major concern, it would be in the best interest of the program to find an optimal solution that gives good results with as little cost as possible. It would be preferable to use cheaper amplifiers due to the fact that amplifiers may break down, so a cheaper amplifier would also provide many replacements.

Amplifiers

There are two different amplifiers we considered for usage in the detector. The LM 741C operational amplifier and the LMH 730154 high speed differential amplifier. Preliminary research indicates that the LM 741C op amp is unsuitable for our purposes. The slew rate is not high enough and the amplification bandwidth is not wide enough.

741C Operational Amplifier

Benefits

The 741C operational amplifier is fairly cheap (approximately $0.75 per chip) and quite durable. It is a popular component for many electronic applications. An operational amplifier can be configured in many ways to sum, subtract, integrate or differentiate signals. However, for our purposes, we need only a non-inverting amplifier.

Drawbacks

Low slew rate and the amplification bandwidth only extends to about 1 MHz. For our purposes, we need an amplification bandwidth of 1 GHz or more is required to amplify the signals coming from the photomultiplier tube.

LMH 730154 Differential Amplifier

Benefits

The slew rate for the LMH 730154 Differential Amplifier is very high and has an amplification bandwidth of approximately 2 GHz. This means that the amplifier can properly amplify signals of up to 2 GHz without any major signal distortions. The photomultiplier tubes produce pulses in the nanosecond range, thus a bandwidth in the gigahertz range is necessary.

Drawbacks

The LMH 730154 Differential Amplifier is quite expensive and very fragile. It is approximately $10 per chip. Electrostatic charge collected on the body can be enough to blow out the solid state electronics packed within the chip. This chip is quite fragile and quite small, extreme car must be taken in order to ensure that the chip remains working.

Photomultiplier Tubes (PMT)

Basic Operation Principle

A photomultiplier tube (PMT) is a device that has the capability of detecting single photons and small pulses of light. The underlying principle governing the workings of the device is the photoelectric effect. The photoelectric effect is simply the action of a photon (with sufficient energy) knocking an electron out of orbit. This electron is then multiplied by a series of plates called dynodes. The dynodes are at a positive voltage so that the electron is accelerated towards the plate. When the electron collides with the dynode a shower of electrons are emitted. These electrons then are accelerated towards the other dynodes and it also emits a shower of electrons. This avalanche effect results in thousands and sometimes millions of electrons being liberated. These are the electrons that are detected as an electrical pulse.

PMT Gain

The gain of a PMT is directly proportional to the voltage applied to the dynodes and the number of dynodes. The gain of a PMT usually ranges from one million to one hundred million. This large gain is sometimes large enough to be detected directly, however tests will have to be run in order to test if this claim is true. If the gain is sufficiently large, then no amplifier is required. If it is not, an amplifier will be required.

Type of PMT

Currently we are testing the Hamamatsu R4125HA photomultiplier tube. The PMT gain was listed as 8.7 x 10^5. It has a maximum operating voltage of 1500 volts. However internet research indicates that operating the tube at 200 to 300 volts lower than the maximum rating is optimal. Some preliminary tests have already been run. We operated the tube at 1100 volts and placed the tube in a dark box. The dark box had a small opening and an LED was placed at the end of the darkened window. The red LED was given 10 millisecond pulses at 2.35 volts. We noticed a signal of 10 mV with a background noise of 3-5 mV.

Procedure

We will begin by testing the PMT without an amplifier to see if we can get a discernible signal consistently, which could potentially allow us to skip using an amplifier altogether. If this course of action does not pan out, we will move on to testing the PMT with both amplifiers and comparing the data. This should help us see which amplifier is the right fit for this usage.