Preamplifiers in radiation measurement systems

[Kevin Ferradas]

In the radiation detection, it's of common use pulse-type systems, which have a preamplifier between the detector and the amplifier. I have read that the preamplifier primary function is reduction of attenuation of the signal that exits from the detector by matching the impedance of the detector with that of the amplifier. How does this matching helps to achieve this goal?

 

[gleem]

I'm not an electronics expert but let me start and others may correct my response if I'm inaccurate.. Impedance matching allows maximum power transfer from one device to another. If the devices are not matched then particularly at high frequencies as contained in pulses the signal is reflected back to the sending device.

I was always under the impression that a preamp's primary function was to produce a better signal to noise ratio by amplifying the detector signal before sending it out over a usually long coax.

Also don't amplifiers typically have high input impedance and relatively low output impedance with the coax having its own impedance typically under 100 ohms ? My impression is that in pulse circuits the actual matching occurred at the end of the coax at the amplifier by using a Tee connector with a terminating resistor of the same impedance as the coax.

 

[Bigjoemonger]

I only took a couple nuke engineering labs on this so my knowledge is not the best but as I recall the general purpose of the pre-amp is to better prepare the signal for it's transfer to the amp. Such as flipping the polarity and bumping up the amplitude for a better signal to noise ratio. The detector doesn't produce a very good signal and traveling along a long wire to the amp can degrade the signal such that its lost in the noise. So the pre-amp is very near or attached to the detector. What exactly it does is dependant on what kind of detector it's attached to and what information you're trying to collect. Whether you want a specific energy in the pulse or you're just looking for time between pulses. But the general idea of it all is it prepares the pulse so that the desired information can make it to the amp. Then the amp shapes the pulse to match the electronics of the equipment reading it.

 

[SAXSIST]

Well, Bigjoemonger explaind already very well. One addition from my side: preamps do what they are designed to do (unlike politicians). But any kind of electronic circuit you may have in mind, never expect a S/N increase, it is quite the opposite!