FAQ - Rolling your own Neutron Counter

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Richard Hull
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FAQ - Rolling your own Neutron Counter

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FOR THE NEWBS........ A geiger counter can be made to detect neutrons as well as everything else. An ion chamber counter can be made to detect neutrons as well as everything else. A scintillation counter can be made to detect neutrons as well as everything else. Thus, the above types of radiation detecting instruments will not help you count neutrons, exclusively. You will have to have a good "neutron specific" counter if you are planning on doing fusion.

Here is how you can get your neutron counter.......

1. Ebay! You may snag one used and, hopefully, functional. A crap shoot, at best, with prices from $200.00, (an obvious non-worker) to $1500.00 for a warranted good counter with a moderate to small size detection tube. Neutron counters on E-bay are notoriously disappointing for the money spent. Stick your neck out and take your chances.

2. You can always buy a neutron counter new from a reputable manufacturer... However, you must have $2000.00+ on hand. With a new counter, you will not be disappointed. You will have a known good, functional neutron counter with a warranty.

3. You can roll your own from scratch. This is probably the cheapest way , but requires you to be somewhat adept at scrounging and knowledgable in basic electronics or at least able to assemble align and test blocks of finished sub-assemblies. No pricing is offered here as it is up to your abilities to grab the right parts at a right price for your purse.

It is only this last #3 item that we will discuss here.

Making your own neutron counter from scratch is basically a great idea. In many cases, it is your only choice.

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The advantages to rolling your own neutron counter are many.

Your have built or assembled and tested it which means you can fix it..........
Based on scrounging and electronics skills, you might be able to save a bundle.............
You can custom make the instrument controlling both size and form to suit your specific needs...............
You will learn certain basics of nuclear instrumentation that the "ready made" purchaser might never learn................
You can alter the design at will, upgrading lesser, earlier components on the fly.......
You can have the pride of accomplishment in succeeding in a relatively complex, involved and difficult task.

What is involved?

There are seven basic building blocks needed to make a classic neutron counter.

1. detector
2. high voltage power supply
3. preamplifier
4. amplifier
5. discriminator
6. counter
7. moderator

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1. Detector - Approximately 95% of all neutron detectors are proportional, gas filled or special material lined detector tubes. All the detectors listed below mainly detect only slower thermal neutrons ~.02ev. Fusion produces only fast neutrons, thus, the need for a moderator as listed above. More on this later.

a. BF3
b. 3He
c. Boron lined
d. Fission tube
e. Scintillation based

The BF3, (boron trifluoride), and 3He, (helium 3), tubes are gas proportional tubes and require very high voltages to be applied, (1200-3500 volts). More on this later. A proportional detector requires special preampliers that are usually a charge sensitive type. More on this later, as well. The vast majority of neutron detection systems are based on these two tubes.

3He tube - Due to a recent shortage of 3He gas, the 3He tubes are virtually no longer found for sale either new or used. When found, they are extremely expensive $500-$2000 with used prices rivaling what new tubes cost when they were sold freely! Why? The 3He tube is, with litle argument, the most sensitive neutron detection device on the planet. It is insensitive to all other radiations, even in rather intense mixed fields of radiation the 3He tube will detect only neutrons. All 3He tubes contain multiple atmospheric pressures with most being at 3 atm. If you can obtain one of these tubes, you are far out in front of the pack.

BF3 tube - More neutrons counters have been made with this type of tube than any other form. Their use goes back to the late 1930's and early 40's. BF3 gas is plentiful and cheap, however, the BF3 tubes are not cheap, if purchased new. Used BF3 tubes abound compared to used 3He tubes. They can sometimes be found for under $100.00 for smaller tubes.

Like the 3He tubes, the BF3 tubes can have pressures from 1/2 atm to 2atm, but a wider range of pressures is common. The BF3 tube is a tiny bit less discriminating in intense gamma ray fields mixed with neutrons than the 3He tube, but is still considered excellent. In this non-extant 3He tube world of 2010, the BF3 tube is probably what many amateurs will have to settle for.

Boron lined tube - This was, at one time, a popular neutron detector, especially around reactors with large neutron fields and limited gamma radiation mixed in. It is not as discriminating or as effecient as the gas proportional tubes. Here, the tube is lined with elemental boron. Some tubes are lined with isotopically enriched B10. A drug on the used market, which is about the only source for these tubes at amateur prices, is the older 1950's General Electric boron lined tube. It can often be found on E-bay for under $100.00. This tube requires far less input voltage which must be critically determined and set. (400-800 volts). Discrimination of non-neutron radiation in this tube is tricky, but many amateurs have found this tube useful and turned it into useful, usable counters.

Fission Tube- This is an almost never encountered tube. You can't really buy one new. Nonetheless, fission tubes have been found used in very rare instances. The classic fission tube contains a tiny amount of plutonium or other fissionable material applied to the walls or a plate within the tube. As such, the sale of these tubes is highly controlled. The most encountered fission tube is a fast neutron detector version lined with larger amounts of U-238. All of these lined or coated tubes are not very efficient. You might foget the fission tube for amateur purposes due to rarity and poor performance.

Scintillation based detector - This detector is a real winner if you sink some money into a special plastic scintillator that is custom designed to detect neutrons while, at the same time, rejecting other forms of radiation that typically do activate scintillation detectors. The Bicron BC-720 neutron scintillator comes in a number of diameters. The most common used size is 2-inches. this very specially constructed device costs about $300.00 and must then be optically coupled to any easily obtained PMT, (photomultiplier tube). 2- inch PMTs with mated preamps can be had for as little as $30.00, thus, the detector and the preamp are handled in one transaction. The PMT also uses high voltage and requires about 900-1100 volts.

This form of detector is considered a "fast" neutron detector meaning that item #7, the moderator, is not normally needed as the plastic composition the the scintillator acts as the moderator to a degree.

A counter made with a scintillation detector is very ruggged, but not particularly efficient. (0.5% is the norm for this type detector - 1 neutron detected for each 200 neutrons entering the scintillator.) There are also some far less useful and less discriminating scintillation schemes such as the simple "hornyack button", or simple scintillation loaded plastic.

2. High Voltage Power Supply

This is most often picked up as a "block" part in the form of a finished NIM component, (nuclear instrument module). Nim modules are regularly offered in quantity on E-bay for as little as $30.00. **Note** All NIM modules are powered by and require what is called a NIM "crate". This is a standard, 19-inch rack mountable cage or bin that will accept a number of different NIM compatible modules. It also contains all the DC power supplies needed to power the NIM modules within the cage.

It is higly recommended to go the NIM route for all of the above block items from #2 thru #6 are to be found ready to go as finished building block items and sold on E-bay. Each NIM module is "signal" compatible with all other NIM modules. Consider NIM as a nuclear instrumentation "erector set" or "mechano" for the Euros.

If you wish to bull head your own HV power supply then you will need to construct a variable 0-2500 volt powr supply that only need supply as little as 2 milliamps. The better supplies must be fitlered, ripple free and stable.

3 - Preamplifier

This item is key to good performance. Nim modules that are charge sensitive preamps are available, but are among the more illusive of the NIM modules. They are easily destroyed, as they have a sensitive FET "front-end" transistor. Few E-bay sellers can test them to warrant operational readiness. Still, try and obtian this item as a NIM module, if possible. These can go from $50.00 to $200.00 when found on E-bay.

For those that have an electronic background, there are old files in this forum by Richard Hester and others that speak to the construction of charge sensitive preamps and circuits are supplied.

4 - The Amplifier

This item is also best located as a NIM module, but any fast amplifier will do for this simple function. Amateur construction is easy and relatively simple if you wish to bull head this issue as well. NIM amp modules are about $40.00 and can also contain useful pulse shaping circuitry.

5- The Discriminator

This item can often be found as part of a key nim module called an SCA, (single Channel amplifier). An SCA can supply both an amplifier and a windowed signal adjustment control group needed to reject or "discriminate" between real neutron signals and noise. Thus, both #4 and #5 are handled in a single unit. Needless to say, once again, it is this item that you really want. NIM SCAs can cost from $25-$100 GO NIM!

6 - Counter

Again, this is a ready to rock and roll item found as a NIM module that is pretty much plug and play. However, any simple electronic digital counter or frequency counter that has a "count mode" will work. **NOTE** Nim counter modules also have a handy built in timer that can supply an automatic or pre-set timed count. Working NIM counters cost about $100.00 or less.

7- Moderator

As noted above most all neutron detectors are thermal or slow neutron detectors and we encounter only fast neutrons in our fusion quest. We need to slow down or "moderate" the neutrons. This can be done with any hydrogenous material such as water, paraffin wax, or polyethylene.

Neutrons much be moderated before they hit the detector tube. This means the tube must be surrounded by the moderator.

A tank of water of can be used with the detector tube placed in the tank such that at least 3" of water is all around the tube.

A large 6inch or 8-inch diameter cylinder of wax or UHMW, (ultra high molecular weight), polyethylene with a bore hole in the center large enough for the tube to be placed within it will serve nicely and be more portable than a water moderated detector.

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Summary

There you have it. You connect all of the pieces in linear order 1 thru 7 and you will have a neutron counter.

There remains the issue of adjustment of the connected components. No better "how to" on this process exists than Carl Willis' epic video on youtube. While Carl shows his process using a 3He tube, the process is little altered for either a BF3 or Boron line tube based system. Check out his posted FAQ in this forum at..........

viewtopic.php?f=13&t=6053&hilit=FAQ#p34497

This post seeks to help a would-be, "roll your own neutron counter" person who either can't find a used counter at a price they like or can't afford a new counter. it is not a hand holding session with plans or diagrams, but an overview that should get the go getters "out of th' chute"

Much of this info that is far more detailed is scattered around this forum on this effort, but this is the first to address a "roll your own" neutron counter. Many FAQS in this forum address or touch upon some few of the above issues in vastly greater detail. Search this good info out and use it to ROLL YOUR OWN.

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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