Question on 3He detector Voltage

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Dennis P Brown
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Question on 3He detector Voltage

Post by Dennis P Brown »

A bit confused - I decided to measure the voltage output on the 3He tube detector box (power supply & USB interface unit) and it read only 317 volts DC. That isn't exactly very high voltage but the device uses RPBNC high voltage connectors. So, is that a typical operating voltage for a 3He detector tube or is the supply unable to be measured by a voltmeter by being too high a load?
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Liam David
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Re: Question on 3He detector Voltage

Post by Liam David »

I'm not sure what detector box you're using, but it sounds like your voltmeter is loading the supply, especially if it's powered over USB. For 3He tubes one would expect the voltage to be 1000 - 2000V, and rarely higher. Mine are set at 1200 and 1400.
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Dennis P Brown
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Re: Question on 3He detector Voltage

Post by Dennis P Brown »

Thank you. Makes a lot of sense since the cables are special high voltage ones.
Peter Schmelcher
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Re: Question on 3He detector Voltage

Post by Peter Schmelcher »

I can’t find the old book on He3 tube design but from memory the tubes have 3 operating regions as the voltage is increased and 317V seems low to me. I like the middle proportional plateau region where the current gain of the detection process is stable and the pulse height is proportional to the initial kinetic energy of the particle plus the He3 decomposition energy. The voltage range depends on the quenching gas and tube dimensions but with the old Russian CN19H tubes it was something like 800-1500V, I choose 1000V. Above 1500V is the start of the Geiger region where the detection process produces a constant pulse height.

With proportional tube operation you can verify that the detection setup is working using cosmic ray created neutrons and a storage oscilloscope. The initial velocity energy entering the tube can be removed by passing the neutrons through a moderator. The moderator vs no-moderator difference is easy to see.
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Richard Hull
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Re: Question on 3He detector Voltage

Post by Richard Hull »

I showed how to convert a $7.00 Harbour Freight 1 meg input DVM to a 0-2000volt, 100 megohm input DVM set on the 20 volt range in an old post.
You absolutely will load a bias supply for a PMT or any proportional detector. Such supplies are designed to never deliver more than 100-200 microamps of current at the high voltage. Regular service DVMs as well as old vacuum tube voltmeters are no good for such measurements.

So weak are bias supplies that, even in some GM supplies, the manufacturer in old service manuals demanded that you set the HV using an electrostatic voltmeter!! 100 megohms of input resistance will not unduly load most bias supplies.

In my case, the maximum draw on my large 5 inch NaI:Tl PMT for use with my gamma spec at 1000volt is only 25ua.

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 3He detector Voltage

Post by Dennis P Brown »

Thank you all for those valuable bits of information. I figured something was amiss but my ignorance on electronics and neutron detectors in particular is too large to enable me to be certain of the correct state of affairs.
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Re: Question on 3He detector Voltage

Post by JoeBallantyne »

If you don't want to pursue Richard's HF multimeter idea you can also just buy this:

https://www.ebay.com/itm/251967239577?h ... SwMKpUZnm0

No relation to the seller.

That device should allow you to measure the voltage with any electronic voltmeter with a 10Mohm input impedance.

Joe.
Last edited by JoeBallantyne on Wed Feb 16, 2022 5:01 pm, edited 3 times in total.
JoeBallantyne
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Re: Question on 3He detector Voltage

Post by JoeBallantyne »

The above listing is just a very high impedance voltage divider which is exactly what Richard's HF multimeter setup gets you. In Richards setup the Harbor Freight multimeter itself is the bottom resistor in the divider.

Joe.
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Jim Kovalchick
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Re: Question on 3He detector Voltage

Post by Jim Kovalchick »

I'm a big fan of using electrostatic voltmeters. With patience one can usually be found on Ebay in the right range for a reasonable price. I have a couple Sensitive Research boxes that I use for my tinkering.

Jim K
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Richard Hull
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Re: Question on 3He detector Voltage

Post by Richard Hull »

I also have those old electrostatic voltmeters one is marked for the AEC ownership.

if you take a normal ohmmeter and the Harbour Freight meter with the battery removed, and read the ohmic resistance on all ranges on the HF meter it is 1 megohm, thus the need for the large external resistor. 1 meg meter input resistance is just fine for 99.9% of all electronic measurements. Even VTVMs of old had 10 meg input resistances and just were not suitable for bias adjustments. Rarely does any DVM or VTVM have an upper range of more than 1500 volts.(VTVMs) Most DVMs top out at 1000 volts at best due to the confined interiors and PC board range switching. The new HF DVM meters no longer go to 1000 volts, but retain the 1 meg input resistance. This in no way impacts their use for the 2000 volt conversion as it makes use of the 20 volt range only.

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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