Question on PreAmp

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Dennis P Brown
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Question on PreAmp

Post by Dennis P Brown »

Just got back from the West coast and discovered the used preamp I ordered, arrived. By the way, took a Red Eye flight back and have been up for over 33 hours now (reminds me of grad school days but I digress).

This is a Ortec 113 preamp. I am testing the unit to see if it will respond to an input pulse and it appears it does (inputted into its test connector). I include a scope capture image, pic of the preamp unit, and overall system pics. I used a square signal pulse (100 Hz) into the unit's test port. The upper line on the scope screen is obviously the square pulse. It is 200 mV per Div. The scope is set to trigger on the leading edge of the preamp output.

The lower display is the preamp's output. It is using 50 mV per Div.

The knob on the unit allows other cap values (with zero setting the only one that appears to provide an output pulse. Really no idea what this is suppose to do or be useful for.)

Further, I also have no idea if this unit is operating correctly; the "preamp" is creating a response to each square wave input pulse. The preamp's output pulse (reading off the scale) peaks at 2.5 Div or only about 125 mV - hence, no real net amplification compared to the square wave input pulse. Since I have no experience with these units I do not know if that is typical for such a preamp or if it just isn't providing any amplification - and is defective.

Of course, if it isn't working correctly, then I need to test that the course amp to determine if it is providing the proper voltage to the pre-amp. But this is my first pass just to see what's what- I'm too tired to get into details with the units. Do note that the course amp does have written on it that it works with any Ortec preamp (as well as other companies units.)
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OrTec PreAmp
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Liam David
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Re: Question on PreAmp

Post by Liam David »

I am not all that familiar with these external units, but it seems to be behaving mostly correctly.

Per the product webpage, the input capacitance is 45 pF with the selector knob allowing you to add up to 1000 pF for detector matching.
From the manual:

For each volt input an output voltage
of approximately 135 mV should be obtained when
an input capacity of 100 pF is selected by S1. The
output pulse should decrease in amplitude as the
input capacity is increased.

The lack of amplification isn't an issue. The pulse height output should be (gain [in volts/coulomb] * test input capacitance * square wave pulse height), which I will write as P = G*C*V. Given the paragraph above, we obtain G = P/CV = 0.135 V/((100 pF + 45 pF)*1 V) = 0.931 mV/pC. Hence, for your settings, we get P = 0.931 mV/pC * 45 pF * 300 mV = 12.5 mV. Somewhere I seem to have missed a factor of 10 which you'll have to find, but 125 mV is spot-on to what you measured.
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Richard Hull
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Re: Question on PreAmp

Post by Richard Hull »

This is a Scintillation pre-amp. You will, most likely have to bias and protect the input externally using a 3He or BF3 detector, using a couple of resistors and a .001 3kv capacitor.

Most all PMTs of manufacture have this taken care of in the rear of its shell where there is an SHV BNC and a safe signal output to a scintillation pre-amp. Be very careful here or you will blow the input!

This is much like the CR-110. It needs external components to bias the tube with a high voltage input cap.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Dennis P Brown
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Re: Question on PreAmp

Post by Dennis P Brown »

Thank you both, Liam and Richard for that information. Having this type of input is extremely useful and essential to people like me that are rather limited in their knowledge. Richard, that is a good catch and I appreciate that important warning and info on what to do - blowing that detector is the last thing I need to do right now.

Forgot to say, thank you Liam for that calculation - I didn't know that. Also, glad to know the unit is working as it should. Definitely will modify it so I can use this unit.

Next step is to open it and see if there is room to add a 3 kV cap and the required Meg-ohm resister. If not, I'll need to add a HV coupling anyway, so maybe just make an aux box for that purpose. I currently have a lot of 6 kV small caps, but thought I still had one 3 kV.
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Richard Hull
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Re: Question on PreAmp

Post by Richard Hull »

The higher the voltage the better! it boils down to space. A .001 uf 250,000 volt cap would work just as well and no better than a 3k, 5kv, 6kv, etc..
for a 3he tube 3kv would be a minimum voltage requirement.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Dennis P Brown
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Real name: Dennis Brown

Re: Question on PreAmp

Post by Dennis P Brown »

Some days I wonder if I can be trusted alone with any complex (i.e. delicate) electronic devices ... wish I was joking.

Thanks again Richard.

I have like half a dozen of those 6 kV, 0.001 uf caps. So, that's well and good. Also, have ten 10 M-ohm resisters and should be able to pick up another (all small 1/10 watt at best.) So, I should be able to install a voltage bias similar to the CR-110 in order to protect the Ortec preamp. I intend to build an add on (RF shielded) box with proper 5 kV and 1 kV HV connectors for that purpose.
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