FAQ - Neutron detector preamplifiers

[Richard Hull]

We have discussed this a bit over many posts, but here goes a sort of mini FAQ on this most needed and hypercritical item.

It is my hope that Richard Hester will join in here and share his thoughts as well for a FAQ is a fabric composed, not only of the original post, but by all replies as well. Richard is uniquely postioned to contribute here. Naturally others will, no doubt, ask good questions and offer other advice.

For all Proportional and most Scintillation detectors, a CHARGE SENSITIVE preamplifier is a must have item if you plan to assemble you own counter using a common neutron detector device.

Proportional detectors - He3, BF3 and Boron lined detector tubes

These neutron detectors are rather expensive and few amateurs can afford to purchase a new detector device. Most new and used detector tubes fall into the hands of amateurs off of E-bay or from surplus sales and hamfests. Most range in price from a few dollars (very rare) to over $400.00each. The condition of these "bargain" detectors is rarely known and can't be known until tested in a properly assembled circuit with a known neutron source.

The output from each is a collection of charge delivered in a pulse and the "front end" of most of the best Charge sensitive amplifiers is a very easily destroyed, low input capacitance FET transistor.

The best charge sensistive amplifiers are NIM related products which, oddly, aren't designed to go in a NIM bin. They do all receive power from the bin via a bin's front panel mounted 9 pin "D" connector or from a NIM module's 9 pin connector which is always found on the back of most spectroscopy amplifiers.

The preamp is usually a small box with a detector input, and a bias voltage input. This allows the box to be located, ideally, very close to the detector tube so that cable capacitance and pickup noise can be kept low via a short cable.

The bias input connector is almost always a special SHV-BNC female. The bias voltage for proportional detectors can easily range from as low as 900 volts to as high as 3500 volts! This bias is current limited and capacitively isolated from the FET gate all inside the preamp. All of this avoids a hassle for the user in locating and installing external hi-ohm, hi-volt limiters and capacitors.

The NIM based preamp is pretty much a plug and play device. There is a warning on many such preamps that you must wait 5 minutes after power down before disconnecting any cables. This allows the charge on any capacitors and cables in the front end to bleed off. If you were to accidientally short the input to ground even with the power just turned off, you could kill the FET front end.

There is also a test input and a pre-amp output BNC on the Nim preamp box. The test terminal lets you inject a low level signal to mime the proportional tube input for adjustment of a following NIM amplifier and also to check out the state of the FET front end.

In general, A NIM charge sensitive preamp has a fixed gain of between 20 and 100 with the higher figure being the most common. Normal output for a BF3 or He3 pulse from a preamp is on the order of 100-500mv. This is usually referenced to the standard 94 ohm NIM output impedance. This signal is then sent via a longer cable to a spectroscopy or linear shaping amplifier where the signal is again boosted based on a variable gain control into the 3-10 volt pulse range. This is then sent to other modules as needed to further descriminate, count and condition the pulse.

Scintillation detectors - Hornyac buttons, ZnS:AG proton recoil systems.

Photomultipliers with Neutron detecting scintillators usually put out a bit more robust signal than the proportional tubes. These do not require the rather bizarre bias treatment that the other tubes require and a much simpler pre-amp can be used.

Richard Hester has designed the "pathetically simple Charge sensitive preamp" found in the files forum, and it is a very servicable item.


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He and I have agreed that the feedback capacitor, (C1- 27pf), especially for use with the He3 and Bf3 tubes, will need to be very small. Most NIM amps use a 1-2 pf 3000 volt special silver ceramic for this purpose. So if you build this preamp, you might decide to experiment with a less than 10pf HV capacitor in this part of the circuit. (Remember, this should be a 3000 volt capacitor.)

Richard will, no doubt, have a bit more input on this.

It is most important to see to it that your preamp is properly installed and that voltages and cables are stable and properly applied to avoid damage to the front end.

E-bay often offers NIM preamps of unkown status for prices ranging from $20.00 to a bid war high of over $150.00! The brand names to look for would be Ortec, Canberra, and Tennelec. (Tennelec is famous for non-standard 9 pin "D" connections so beware). These NIM preamps are relatively easily repaired with a modern FET which is the most common failure item. It will be up to you to work all this out on your own.

I do know that a working, tested and warranted preamp is easily worth over $100.00 used. So let the buyer beware. Good luck in securing this critical item if you are planning on rolling your own neutron counter from a raw detector.

Richard Hull