What is the best preamp for a He3 tube

[pfostini]

Now that I have a good He3 tube I am now looking to purchase/make a preamp for it. In Richards post he mentions a Princeton Gamma and an ortec 122S. These preamps differ from what I have ( all scintillator types ), what moels will work for He3 detectors. The Ametek site has model 142's and when last checked Don Orie was out. Any help on the models I should look for would be appreciated. My tube voltage is up to 5KV. All other components are Nim based.

[Richard Hester]

Do you want to build a peamp or buy one? A pretty suitable preamp could be built with 4 active devices (3 transistors, 1 JFET), but the one piece of the puzzle needed is the average charge in picocoulombs per pulse from the He3 tube. This would be used in selecting the value for the charge storage capacitor for a charge sensitive amplifier. If this is not available, one could still select an appopriate value via cut and try, starting with 1-2pF and working upward.The biggest problem might be finding a coupling cap with a suitable voltage rating (6kV+). The whole thing could be constructed on a piece of perf board. Just about all the parts could be bought from Radio Shack.
After seeing the ratty printed circuit board from the guts of an old Ortec 109, I am no longer ashamed of any of my efforts toward constructing a charge sensitive amplifier. I've etched better boards in my kitchen sink....

[pfostini]

Building is good. Any schematics for such an animal?

[AllenWallace]

Your tube voltage is "up to 5 KV" ? What pressure is the tube?

I have two tubes, one rated at 1800 VDC (with an intregrated power supply-preamp-SCA) and the other tube is rated at 3750 volts and is a time-of-flight device. Your high voltage makes me suspect your tube might be a TOF device.

What difference does this make? I'm not sure, but I suspect that the preamp must work with fast pulses.

[Richard Hester]

If you are just looking to detect neutrons, not do TOF, and you use a charge sensitive amp, a lot of the concerns about needing an obscenely fast preamp just melt right away..... Check out the post called "charge sensitive amplifiers" in the files section, and you find three circuits, any one of which should serve your needs. Some tinkering will be needed for the values of the charge storage cap and reset resistor, but I can advise regarding that.

[Starfire]

A old method fo making lo-value trim caps ( few pf ) was to twist a pair of wires and then trim - wire with 10kv ptfe insulation is easy got - a twist of about an inch of wire will give approx 10 pf -depending upon the twist. It works at a pinch and is easy adjusted - when you get the best value, a touch of nail varnish will fix it.

[pfostini]

The tube was purchased from Don Orie. I was told they can run up to 5KV.. It is the same size as Richard Hulls. I looked at the schematics but did not see any that has the HV bias input for the tube. Is this easily added in?

[Richard Hull]

Yes, it is easily added.

You will need a .005uf cap at 5kv and a 100meg HV resistor.

The tube bias goes on the 100meg resistor. The other end of the resistor goes to the capacitor and the other lead on the cap goes to ground. the cap resistor junction goes to the tube's center wire.

You will need a .001uf cap at 5kv to couple from the tube to the input of your preamp circuit.

Finally, you might inch up to that 5kv rated voltage. I bet you start seeing neutrons at 1500-2000 volts or so.

Richard Hull

[csmith]

Hi all,
I also have a He3 probe from SG. It is rated to be biased at 1.2KV. The charge per pulse at the operating voltage is approx. 0.25pC. What would you suggest the cap and resistor values should be to get a decent size pulse from this tube?

Thanks
Conrad Smith

[pfostini]

Thanks all , I think I got it.